shortlog: default to HEAD when the standard input is a tty

Instead of warning the user that it is expecting git log output from
the standard input (and waiting for the user to type the log from
the keyboard, which is a silly thing to do), default to traverse from
HEAD when there is no rev parameter given and the standard input is
a tty.

This factors out a useful helper "add_head()" from builtin-diff.c to a
more appropriate place revision.c while renaming it to more descriptive
name add_head_to_pending(), as that is what the function is about.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-diff.c

@@ -176,18 +176,6 @@ static int builtin_diff_combined(struct rev_info *revs,
 	return 0;
 }
 
-void add_head(struct rev_info *revs)
-{
-	unsigned char sha1[20];
-	struct object *obj;
-	if (get_sha1("HEAD", sha1))
-		return;
-	obj = parse_object(sha1);
-	if (!obj)
-		return;
-	add_pending_object(revs, obj, "HEAD");
-}
-
 static void refresh_index_quietly(void)
 {
 	struct lock_file *lock_file;
@@ -272,7 +260,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 			if (!strcmp(arg, "--"))
 				break;
 			else if (!strcmp(arg, "--cached")) {
-				add_head(&rev);
+				add_head_to_pending(&rev);
 				if (!rev.pending.nr)
 					die("No HEAD commit to compare with (yet)");
 				break;

builtin-log.c

@@ -18,9 +18,6 @@
 static int default_show_root = 1;
 static const char *fmt_patch_subject_prefix = "PATCH";
 
-/* this is in builtin-diff.c */
-void add_head(struct rev_info *revs);
-
 static void add_name_decoration(const char *prefix, const char *name, struct object *obj)
 {
 	int plen = strlen(prefix);
@@ -746,7 +743,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			 * does not have.
 			 */
 			rev.pending.objects[0].item->flags |= UNINTERESTING;
-			add_head(&rev);
+			add_head_to_pending(&rev);
 		}
 		/*
 		 * Otherwise, it is "format-patch -22 HEAD", and/or

builtin-shortlog.c

@@ -249,9 +249,10 @@ int cmd_shortlog(int argc, const char **argv, const char *prefix)
 
 	read_mailmap(&mailmap, ".mailmap", &common_repo_prefix);
 
+	/* assume HEAD if from a tty */
+	if (!rev.pending.nr && isatty(0))
+		add_head_to_pending(&rev);
 	if (rev.pending.nr == 0) {
-		if (isatty(0))
-			fprintf(stderr, "(reading log to summarize from standard input)\n");
 		read_from_stdin(&list);
 	}
 	else

revision.c

@@ -139,6 +139,18 @@ void add_pending_object(struct rev_info *revs, struct object *obj, const char *n
 	add_pending_object_with_mode(revs, obj, name, S_IFINVALID);
 }
 
+void add_head_to_pending(struct rev_info *revs)
+{
+	unsigned char sha1[20];
+	struct object *obj;
+	if (get_sha1("HEAD", sha1))
+		return;
+	obj = parse_object(sha1);
+	if (!obj)
+		return;
+	add_pending_object(revs, obj, "HEAD");
+}
+
 static struct object *get_reference(struct rev_info *revs, const char *name, const unsigned char *sha1, unsigned int flags)
 {
 	struct object *object;

revision.h

@@ -130,6 +130,8 @@ extern void add_object(struct object *obj,
 
 extern void add_pending_object(struct rev_info *revs, struct object *obj, const char *name);
 
+extern void add_head_to_pending(struct rev_info *);
+
 enum commit_action {
 	commit_ignore,
 	commit_show,