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

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

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