use strip_suffix and xstrfmt to replace suffix

When we want to convert "foo.pack" to "foo.idx", we do it by
duplicating the original string and then munging the bytes
in place. Let's use strip_suffix and xstrfmt instead, which
has several advantages:

  1. It's more clear what the intent is.

  2. It does not implicitly rely on the fact that
     strlen(".idx") <= strlen(".pack") to avoid an overflow.

  3. We communicate the assumption that the input file ends
     with ".pack" (and get a run-time check that this is so).

  4. We drop calls to strcpy, which makes auditing the code
     base easier.

Likewise, we can do this to convert ".pack" to ".bitmap",
avoiding some manual memory computation.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -1511,6 +1511,7 @@ int finish_http_pack_request(struct http_pack_request *preq)
 	struct packed_git **lst;
 	struct packed_git *p = preq->target;
 	char *tmp_idx;
+	size_t len;
 	struct child_process ip = CHILD_PROCESS_INIT;
 	const char *ip_argv[8];
 
@@ -1524,9 +1525,9 @@ int finish_http_pack_request(struct http_pack_request *preq)
 		lst = &((*lst)->next);
 	*lst = (*lst)->next;
 
-	tmp_idx = xstrdup(preq->tmpfile);
-	strcpy(tmp_idx + strlen(tmp_idx) - strlen(".pack.temp"),
-	       ".idx.temp");
+	if (!strip_suffix(preq->tmpfile, ".pack.temp", &len))
+		die("BUG: pack tmpfile does not end in .pack.temp?");
+	tmp_idx = xstrfmt("%.*s.idx.temp", (int)len, preq->tmpfile);
 
 	ip_argv[0] = "index-pack";
 	ip_argv[1] = "-o";