path: expose `do_git_path()` as `repo_git_pathv()`

path: expose `do_git_path()` as `repo_git_pathv()`

We're about to move functions of the "path" subsytem that do not use a
`struct repository` into "path.h" as static inlined functions. This will
require us to call `do_git_path()`, which is internal to "path.c".

Expose the function as `repo_git_pathv()` to prepare for the change.

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