Add implementation and test for discover_path

[mikayla-maki]

Hi y'all,

We're looking to use this library to build the git integration in our code editor (zed.dev). In order to do so, we wanted to cache the Repository::open_ext() call based on the repository path. But git2-rs's implementation of Repository::discover() automatically opens the repository for us, making it impossible to cache :(

I've added a Repository::discover_path() function that more closely matches the libgit2 git_repository_discover() call. I had to add an extra allocation to convert the git buffer into a PathBuf and would appreciate feedback on if there's a way to avoid the allocation.

I also added 2 tests, one for a successful discovery and one for an unsuccessful one that uses ceiling_dirs.

[ehuss]

Thanks!