path: drop `git_path()` in favor of `repo_git_path()`

path: drop `git_path()` in favor of `repo_git_path()`

Remove `git_path()` in favor of the `repo_git_path()` family of
functions, which makes the implicit dependency on `the_repository` go
away.

Note that `git_path()` returned a string allocated via `get_pathname()`,
which uses a rotating set of statically allocated buffers. Consequently,
callers didn't have to free the returned string. The same isn't true for
`repo_common_path()`, so we also have to add logic to free the returned
strings.

This refactoring also allows us to remove `repo_common_pathv()` as well
as `get_pathname()` from the public interface.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>