Add a compile-time option to use the new, very fast sha1dc Rust crate

I stumbled across this new Rust crate last week. Its performance numbers are quite impressive. Naturally, I want to make use of this and get for Windows, which is used on many monorepos where this makes a real difference: In a pretty fast and loose test, I verified that a git index-pack runs roughly three times faster solely due to using those SIMD-based optimizations!

As a safety precaution, because this sha1dc crate is quite new, I wanted to introduce an escape hatch: core.sha1dcBackend=c, but turn it on by default, which is the reason for the three additional patches. Should these patches be undesirable for the Git project? I would not be mad at all if they were simply dropped.

Johannes Schindelin (4):
libgitcore: add sha1dc as an optional feature
sha1dc: allow selecting the C backend without rebuilding
pthread: provide pthread_once() shims for Windows and for
NO_PTHREADS
sha1dc: make sha1dc_init() thread-safe

 Cargo.toml                     |   4 ++
 Documentation/config/core.adoc |   5 ++
 Makefile                       |  29 +++++++++
 compat/win32/pthread.c         |  16 +++++
 compat/win32/pthread.h         |   5 ++
 hash.h                         |   5 ++
 sha1dc_git.c                   | 109 +++++++++++++++++++++++++++++++--
 sha1dc_git.h                   |   5 +-
 sha1dc_rs.h                    |  37 +++++++++++
 src/lib.rs                     |   2 +
 src/sha1dc_rs.rs               |  78 +++++++++++++++++++++++
 t/helper/test-sha1.c           |  19 +++++-
 t/t0013-sha1dc.sh              |  21 ++++++-
 thread-utils.h                 |  16 +++++
 14 files changed, 343 insertions(+), 8 deletions(-)
 create mode 100644 sha1dc_rs.h
 create mode 100644 src/sha1dc_rs.rs

base-commit: 3bc0341126508f78f5869cbfc0005e987efdf0c7
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2240%2Fdscho%2Foptionally-use-sha1dc-rs-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2240/dscho/optionally-use-sha1dc-rs-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2240

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