Introduce libgit-rs, a Rust wrapper around libgit.a

This series provides two small Rust wrapper libraries around parts of
Git: "libgit-sys", which exposes a few functions from libgit.a, and "libgit", which provides a more Rust-friendly interface to some of those functions. In addition to included unit tests, at $DAYJOB we have tested building JJ[1] with our library and used it to replace some of the libgit2-rs uses.

[1] https://github.com/jj-vcs/jj

There is known NEEDSWORK, but I feel that they can be addressed in follow-up changes, rather than in this series. If you feel otherwise, please let me know:

* Investigate alternative methods of managing symbol visibility & renaming.

* Figure out symbol versioning

Changes in V9:
* Properly initialize Makefile var LIBGIT_PUB_OBJS.

* Pass the correct pointer to free in libgit_configset_free().

Changes in V8:
* Define a private libgit_config_set struct to avoid excessive casting in public_symbol_export.c.

* Style fixes: merge some Makefile rules, limit rule line length by defining intermediate variables, add initial empty comment line, add linebreaks in function definitions.

Changes in V7:
* Moved the ConfigSet implementation in libgit-rs to a config module.

* Added doc comments for ConfigSet and its methods.

* Fix meson builds by adding new object files to libgit_sources

* Moved INCLUDE_LIBGIT_RS Makefile changes earlier in the series, so that we can make it optional to compile some of the libgitpub sources. Squashed V6 patch 5/5 into this series' patch 4/4.

* Don't publicly export FFI types in libgit-rs.

* Removed extraneous -r argument to $(RM) in the clean rules.

* Added TODO reminder in Cargo.toml about removing Cargo.lock once we hit a certain minimum supported Rust version.

* Style cleanup in public_symbol_export.c

Changes in V6:
* Rebased onto current master, since V5 was several months old.

* Move libgit-sys out of libgit-rs; while this sort of nesting is common in Rust crates with standalone repositories, it doesn't make as much sense when they're contained in the larger Git project's repo.

* Standardize the naming of some of the Makefile targets to always include a dash in the "-rs" or "-sys" suffixes.

* Clean up READMEs and crate descriptions in preparation for uploading to crates.io.

Changes in V5:
* When building with INCLUDE_LIBGIT_RS defined, add "-fvisibility=hidden" to CFLAGS. This allows us to manage symbol visibility in libgitpub.a without causing make all to rebuild from scratch due to changing CFLAGS.

* Avoid using c_int in the higher-level Rust API.

* Remove libgitpub.a and intermediate files with make clean.

Changes in V4:
* Drop V3 patch #3, which added wrappers around repository initialization and config access. These are not well-libified, and they are not necessary for JJ's proof-of-concept use case, so let's avoid exporting them for now.

* Set a minimum supported Rust version of 1.63. Autodetect whether our Rust version has c_int and c_char types; if not, define them ourselves.

* When building libgitpub.a via build.rs, set DEVELOPER=1 to catch additional errors at build time.

* In build.rs, use the make_cmd crate to portable select the correct invocation of GNU Make.

* Follow naming standards for _alloc() and _free() functions.

* Use String instead of CString in higher-level API.

* Move libgit_configset_alloc() and libgit_configset_free() out of upstream Git, to the libgitpub shim library.

* In libgitpub, initialize libgit_config_set structs in the _alloc() function rather than with a separate _init() function.

* Remove unnecessary comments in libgit-sys showing where the wrapped functions were originally defined.

* Fix clippy lint: don't reborrow configfile path references.

* Various typo fixes and cargo fmt fixes.

Changes in V3:
* Renamed cgit-rs to libgit-rs and cgit-sys to libgit-sys

* Makefile cleanup, particularly adding config.mak options that developers can set to run Rust builds and tests by default (Patch 6)

* Provide testdata configs for unit tests

* ConfigSet API now uses &Path instead of &str -- more ergonomic for Rust users to pass in and errors out if the path string isn't UTF-8

* Fixed unresolved dependency on libz in Cargo.toml

Calvin Wan (1):
libgit: add higher-level libgit crate

Josh Steadmon (3):
common-main: split init and exit code into new files
libgit-sys: introduce Rust wrapper for libgit.a
libgit-sys: also export some config_set functions

 .gitignore                                |   2 +
 Makefile                                  |  50 ++++++++++
 common-exit.c                             |  26 ++++++
 common-init.c                             |  63 +++++++++++++
 common-init.h                             |   6 ++
 common-main.c                             |  83 +----------------
 contrib/libgit-rs/Cargo.lock              |  77 ++++++++++++++++
 contrib/libgit-rs/Cargo.toml              |  17 ++++
 contrib/libgit-rs/README.md               |  13 +++
 contrib/libgit-rs/build.rs                |   4 +
 contrib/libgit-rs/src/config.rs           | 106 ++++++++++++++++++++++
 contrib/libgit-rs/src/lib.rs              |   1 +
 contrib/libgit-rs/testdata/config1        |   2 +
 contrib/libgit-rs/testdata/config2        |   2 +
 contrib/libgit-rs/testdata/config3        |   2 +
 contrib/libgit-sys/Cargo.lock             |  69 ++++++++++++++
 contrib/libgit-sys/Cargo.toml             |  19 ++++
 contrib/libgit-sys/README.md              |   4 +
 contrib/libgit-sys/build.rs               |  35 +++++++
 contrib/libgit-sys/public_symbol_export.c |  59 ++++++++++++
 contrib/libgit-sys/public_symbol_export.h |  18 ++++
 contrib/libgit-sys/src/lib.rs             |  79 ++++++++++++++++
 meson.build                               |   2 +
 t/Makefile                                |  15 +++
 24 files changed, 673 insertions(+), 81 deletions(-)
 create mode 100644 common-exit.c
 create mode 100644 common-init.c
 create mode 100644 common-init.h
 create mode 100644 contrib/libgit-rs/Cargo.lock
 create mode 100644 contrib/libgit-rs/Cargo.toml
 create mode 100644 contrib/libgit-rs/README.md
 create mode 100644 contrib/libgit-rs/build.rs
 create mode 100644 contrib/libgit-rs/src/config.rs
 create mode 100644 contrib/libgit-rs/src/lib.rs
 create mode 100644 contrib/libgit-rs/testdata/config1
 create mode 100644 contrib/libgit-rs/testdata/config2
 create mode 100644 contrib/libgit-rs/testdata/config3
 create mode 100644 contrib/libgit-sys/Cargo.lock
 create mode 100644 contrib/libgit-sys/Cargo.toml
 create mode 100644 contrib/libgit-sys/README.md
 create mode 100644 contrib/libgit-sys/build.rs
 create mode 100644 contrib/libgit-sys/public_symbol_export.c
 create mode 100644 contrib/libgit-sys/public_symbol_export.h
 create mode 100644 contrib/libgit-sys/src/lib.rs

Range-diff against v8:

1:  cd0cb9aa04 = 1:  cd0cb9aa04 common-main: split init and exit code into new files
2:  3588a3c3fc ! 2:  8793ff64a7 libgit-sys: introduce Rust wrapper for libgit.a
    @@ Makefile: $(CLAR_TEST_PROG): $(UNIT_TEST_DIR)/clar.suite $(CLAR_TEST_OBJS) $(GIT
     +all:: libgit-sys
     +endif
     +
    -+LIBGIT_PUB_OBJS = contrib/libgit-sys/public_symbol_export.o
    ++LIBGIT_PUB_OBJS =
    ++LIBGIT_PUB_OBJS += contrib/libgit-sys/public_symbol_export.o
     +LIBGIT_PUB_OBJS += libgit.a
     +LIBGIT_PUB_OBJS += reftable/libreftable.a
     +LIBGIT_PUB_OBJS += xdiff/lib.a
3:  f4452fffe6 ! 3:  ab32bd1d07 libgit-sys: also export some config_set functions
    @@ contrib/libgit-sys/public_symbol_export.c
     +void libgit_configset_free(struct libgit_config_set *cs)
     +{
     +	git_configset_clear(&cs->cs);
    -+	free(&cs->cs);
    ++	free(cs);
     +}
     +
     +int libgit_configset_add_file(struct libgit_config_set *cs, const char *filename)

4: ada9fc0a13 ! 4: 1bf8c5392c libgit: add higher-level libgit crate
@@ Makefile: build-unit-tests: $(UNIT_TEST_PROGS) $(CLAR_TEST_PROG)
+all:: libgit-sys libgit-rs
endif

- LIBGIT_PUB_OBJS = contrib/libgit-sys/public_symbol_export.o
+ LIBGIT_PUB_OBJS =

## contrib/libgit-rs/Cargo.lock (new) ##
@@

base-commit: 757161efcca150a9a96b312d9e780a071e601a03

2.48.1.362.g079036d154-goog