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>

builtin-diff.c

@@ -232,7 +232,7 @@ static int builtin_diff_combined(struct rev_info *revs,
 	return 0;
 }
 
-static void add_head(struct rev_info *revs)
+void add_head(struct rev_info *revs)
 {
 	unsigned char sha1[20];
 	struct object *obj;

builtin-log.c

@@ -11,6 +11,9 @@
 #include "log-tree.h"
 #include "builtin.h"
 
+/* this is in builtin-diff.c */
+void add_head(struct rev_info *revs);
+
 static int cmd_log_wc(int argc, const char **argv, char **envp,
 		      struct rev_info *rev)
 {
@@ -185,6 +188,11 @@ int cmd_format_patch(int argc, const char **argv, char **envp)
 	if (argc > 1)
 		die ("unrecognized argument: %s", argv[1]);
 
+	if (rev.pending_objects && rev.pending_objects->next == NULL) {
+		rev.pending_objects->item->flags |= UNINTERESTING;
+		add_head(&rev);
+	}
+
 	if (!use_stdout)
 		realstdout = fdopen(dup(1), "w");