Add commit-graph fuzzer and fix buffer overflow

Add a new fuzz test for the commit graph and fix a buffer read-overflow that it discovered. Additionally, fix the Makefile instructions for building fuzzers.

Changes since V5:
* Conform to commit message standards for the 1st patch in the series.
* Clarify commit message for the 3rd patch in the series.

Changes since V4:
* Ensure that corrupt_graph_and_verify() in t5318 changes to the proper directory before accessing any files.

Changes since V3:
* Improve portability of the new test functionality.
* Fix broken &&-chains in tests.

Changes since V2:
* Avoid pointer arithmetic overflow when checking the graph's chunk count.
* Merge the corrupt_graph_and_verify and corrupt_and_zero_graph_then_verify test functions.

Josh Steadmon (3):
commit-graph, fuzz: Add fuzzer for commit-graph
commit-graph: fix buffer read-overflow
Makefile: correct example fuzz build

 .gitignore              |  1 +
 Makefile                |  3 +-
 commit-graph.c          | 67 +++++++++++++++++++++++++++++------------
 commit-graph.h          |  3 ++
 fuzz-commit-graph.c     | 16 ++++++++++
 t/t5318-commit-graph.sh | 16 ++++++++--
 6 files changed, 83 insertions(+), 23 deletions(-)
 create mode 100644 fuzz-commit-graph.c

Range-diff against v5:
1: 0b57ecbe1b ! 1: c4ec3fc3fc commit-graph, fuzz: Add fuzzer for commit-graph
@@ -2,11 +2,11 @@

commit-graph, fuzz: Add fuzzer for commit-graph

- Breaks load_commit_graph_one() into a new function,
- parse_commit_graph(). The latter function operates on arbitrary buffers,
- which makes it suitable as a fuzzing target. Since parse_commit_graph()
- is only called by load_commit_graph_one() (and the fuzzer described
- below), we omit error messages that would be duplicated by the caller.
+ Break load_commit_graph_one() into a new function, parse_commit_graph().
+ The latter function operates on arbitrary buffers, which makes it
+ suitable as a fuzzing target. Since parse_commit_graph() is only called
+ by load_commit_graph_one() (and the fuzzer described below), we omit
+ error messages that would be duplicated by the caller.

Adds fuzz-commit-graph.c, which provides a fuzzing entry point compatible with libFuzzer (and possibly other fuzzing engines).

2:  a3b5d33c4b = 2:  d7b137650f commit-graph: fix buffer read-overflow
3:  350ea5f7c9 ! 3:  c06e0667fa Makefile: correct example fuzz build

@@ -2,6 +2,15 @@

Makefile: correct example fuzz build

+ The comment explaining how to build the fuzzers was broken in
+ 927c77e7d4d ("Makefile: use FUZZ_CXXFLAGS for linking fuzzers",
+ 2018-11-14).
+
+ When building fuzzers, all .c files must be compiled with coverage
+ tracing enabled. This is not possible when using only FUZZ_CXXFLAGS, as
+ that flag is only applied to the fuzzers themselves. Switching back to
+ CFLAGS fixes the issue.
+

diff --git a/Makefile b/Makefile

2.20.1.97.g81188d93c3-goog