pathspec: provide a more descriptive die message

The current message displayed upon an internal error in
'init_pathspec_item()' isn't very descriptive and doesn't provide much
context to where the error occurred.  Update the error message to
provide more context to where the error occured.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pathspec.c

@@ -555,7 +555,7 @@ static void init_pathspec_item(struct pathspec_item *item, unsigned flags,
 		 * would trigger that.
 		 */
 		die_inside_submodule_path(item);
-		die ("BUG: item->nowildcard_len > item->len || item->prefix > item->len)");
+		die ("BUG: error initializing pathspec_item");
 	}
 }