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>

builtin/am.c

@@ -158,7 +158,7 @@ static void am_state_init(struct am_state *state)
 
 	memset(state, 0, sizeof(*state));
 
-	state->dir = git_pathdup("rebase-apply");
+	state->dir = repo_git_path(the_repository, "rebase-apply");
 
 	state->prec = 4;