checkout: clarify memory ownership in unique_tracking_name()

The function `unique_tracking_name()` returns an allocated string, but
does not clearly indicate this because its return type is `const char *`
instead of `char *`. This has led to various callsites where we never
free its returned memory at all, which causes memory leaks.

Plug those leaks and mark now-passing tests as leak free.

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

checkout.h

@@ -8,8 +8,8 @@
  * tracking branch.  Return the name of the remote if such a branch
  * exists, NULL otherwise.
  */
-const char *unique_tracking_name(const char *name,
-				 struct object_id *oid,
-				 int *dwim_remotes_matched);
+char *unique_tracking_name(const char *name,
+			   struct object_id *oid,
+			   int *dwim_remotes_matched);
 
 #endif /* CHECKOUT_H */