sha1_file_name(): declare to return a const string

Change the return value of sha1_file_name() to (const char *).
(Callers have no business mucking about here.)  Change callers
accordingly, deleting a few superfluous temporary variables along the
way.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -1384,7 +1384,7 @@ struct http_object_request *new_http_object_request(const char *base_url,
 	unsigned char *sha1)
 {
 	char *hex = sha1_to_hex(sha1);
-	char *filename;
+	const char *filename;
 	char prevfile[PATH_MAX];
 	int prevlocal;
 	char prev_buf[PREV_BUF_SIZE];