path: drop `git_common_path()` in favor of `repo_common_path()`

path: drop `git_common_path()` in favor of `repo_common_path()`

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

Note that `git_common_path()` used to return 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()` from the
public interface.

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