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>

dir.c

@@ -3455,7 +3455,7 @@ void setup_standard_excludes(struct dir_struct *dir)
 
 char *get_sparse_checkout_filename(void)
 {
-	return git_pathdup("info/sparse-checkout");
+	return repo_git_path(the_repository, "info/sparse-checkout");
 }
 
 int get_sparse_checkout_patterns(struct pattern_list *pl)