fmt-patch: understand old <his> notation

When calling "git fmt-patch HEAD~5", you now get the same as if you would
have said "git fmt-patch HEAD~5..". This makes it easier for my fingers
which are so used to the old syntax.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/builtin-diff.c b/builtin-diff.c
index 636edbf..2087316 100644
--- a/builtin-diff.c
+++ b/builtin-diff.c
@@ -232,7 +232,7 @@
 	return 0;
 }
 
-static void add_head(struct rev_info *revs)
+void add_head(struct rev_info *revs)
 {
 	unsigned char sha1[20];
 	struct object *obj;