explore replacing plugins/git nom parser with libgit2 function calls

[j-lanson]

To get commit diffs for a repo, our git plugin currently uses a configured git log message to get the entire diff history the repo, and uses a nom parser to extract data into Diff objects. The log data we parse does not include commit hashes, so if there are any corner cases, the parser will fail and commits.len() != diffs.len(). Rather than relying on the parser, we should see if we can use the Rust libgit2 bindings to synthesize CommitDiff objects in a much more reliable way.

This link shows example code in C of how to use the libgit2 API to get git log -like info. The task will be to see if we can do something similar to get the diff info we need to construct CommitDiff objects, and preferably remove the need to zip two independently-derived Vecs together.