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

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

Remove `git_pathdup()` in favor of `repo_git_path()`. 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>