path: drop `git_path_buf()` in favor of `repo_git_path_replace()`

path: drop `git_path_buf()` in favor of `repo_git_path_replace()`

Remove `git_path_buf()` in favor of `repo_git_path_replace()`. The
latter does essentially the same, with the only exception that it does
not rely on `the_repository` but takes the repo as separate parameter.

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