mailmap: always store allocated strings in mailmap blob

Same as with the preceding commit, the `git_mailmap_blob` may sometimes
contain an allocated string and sometimes it may contain a string
constant. This is risky and can easily lead to bugs in case the variable
is getting re-assigned, where the code may then try to free the previous
value to avoid memory leaks.

Safeguard the code by always storing allocated strings in the variable.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

mailmap.c

@@ -216,7 +216,7 @@ int read_mailmap(struct string_list *map)
 	map->cmp = namemap_cmp;
 
 	if (!git_mailmap_blob && is_bare_repository())
-		git_mailmap_blob = "HEAD:.mailmap";
+		git_mailmap_blob = xstrdup("HEAD:.mailmap");
 
 	if (!startup_info->have_repository || !is_bare_repository())
 		err |= read_mailmap_file(map, ".mailmap",