use libgit2 git_buf instead of (char *, size) for repo path

Commit 2fddc49 (adapt to libgit2 > 0.20.0) made gbr_repo_open()
convert the passed-in (path,size) tuple into a git_buf for
git_repository_discover() and then back again before returning.

We can use a git_buf object for storing `path' from the get-go
instead of translating back and forth.