feat: use libgit2 for parsing git information in mitre/git plugin

[patrickjcasey]

This pull request removes the need for a custom nom-based parser for parsing git information in the mitre/git plugin, by swapping over to using libgit2 for parsing git information. As a part of this change, I got some data from the following repos:

• https://github.com/mitre/hipcheck
• https://github.com/numpy/numpy
• https://github.com/linux/torvalds

for determining execution speed, the following command was run with the release version of hc on my M1 Macbook Pro with 32 GB RAM, the mitre/git plugin was also configured to run the release version:

time ./target/release/hc check --policy config/Hipcheck.kdl <REPO>

for testing main, commit 7b2e5a5629928cc448a7cfee56178370d4f32cc3 was used

Branch	Repo	# of Commits	# of CommitDiffs	# of Contributors	Last Commit	Execution Speed
main	mitre/hipcheck	502	502	19	2024-11-25T07:04:37Z	11.45 s
libgit2	mitre/hipcheck	502	502	19	2024-11-25T07:04:37Z	10.19 s
main	numpy/numpy	28386	28386	1897	2024-11-25T21:22:49Z	4m 40s
libgit2	numpy/numpy	37551	37551	2239	2024-11-25T14:22:49Z	12m 50s
main	torvalds/linux
libgit2	torvalds/linux	1320823	1320823	38849

git rev-list --count HEAD results:

• numpy/numpy: 37751 is returned, which matches the libgit2 implementation
• torvalds/linux: 1320823 is returned, which matches the libgit2 implementation

git log --pretty="%an %ae%n%cn %ce" | LC_ALL=C sort -u | wc -l results:
numpy/numpy: 2239 is returned, which matches libgit2 implementation
torvalds/linux: 38855 is returned, which is off by 6 from the libgit2 implementation