Add mailmap.file as configurational option for mailmap location

This allows us to augment the repo mailmap file, and to use
mailmap files elsewhere than the repository root. Meaning
that the entries in mailmap.file will override the entries
in "./.mailmap", should they match.

Signed-off-by: Marius Storm-Olsen <marius@trolltech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-blame.c

@@ -2394,7 +2394,7 @@ parse_done:
 		die("reading graft file %s failed: %s",
 		    revs_file, strerror(errno));
 
-	read_mailmap(&mailmap, ".mailmap", NULL);
+	read_mailmap(&mailmap, NULL);
 
 	if (!incremental)
 		setup_pager();