facade: use libgit2 for git operations to improve performance and security

> [!NOTE]
> Migrated from [augurlabs/augur#3787](https://github.com/augurlabs/augur/issues/3787)
> Originally opened by `@MoralCode` on 2026-03-20

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Libgit2 ([website](https://www.libgit2.org/), [repo](https://github.com/libgit2/libgit2)) is a cross platform C library implementing the git plumbing. It is designed for use as a library within complex applications, unlike the git CLI which is designed as a CLI. ([source](https://stackoverflow.com/a/22097293))

https://www.pygit2.org/ is the python version of this.

I learned about libgit2 through a conversation in a nerdy group chat of mine where there was a large 80ms -> 0.5ms speedup for one minimal usecase (render times for displaying git repo status in your shell prompt). While this may not be applicable to Augur directly, I wonder if we could eliminate a lot of shell calls, parsing bugs, and extra layers in the process when facade collects a git repo by using git more directly via a library that directly interfaces with git's plumbing, rather than calling an external program to do it.