fsck: --no-dangling omits "dangling object" information

The default output from "fsck" is often overwhelmed by informational
message on dangling objects, especially if you do not repack often, and a
real error can easily be buried.

Add "--no-dangling" option to omit them, and update the user manual to
demonstrate its use.

Based on a patch by Clemens Buchacher, but reverted the part to change
the default to --no-dangling, which is unsuitable for the first patch.
The usual three-step procedure to break the backward compatibility over
time needs to happen on top of this, if we were to go in that direction.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/fsck.c b/builtin/fsck.c
index 8c479a7..67eb553 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -29,6 +29,7 @@
 static int write_lost_and_found;
 static int verbose;
 static int show_progress = -1;
+static int show_dangling = 1;
 #define ERROR_OBJECT 01
 #define ERROR_REACHABLE 02
 #define ERROR_PACK 04
@@ -221,8 +222,9 @@
 	 * start looking at, for example.
 	 */
 	if (!obj->used) {
-		printf("dangling %s %s\n", typename(obj->type),
-		       sha1_to_hex(obj->sha1));
+		if (show_dangling)
+			printf("dangling %s %s\n", typename(obj->type),
+			       sha1_to_hex(obj->sha1));
 		if (write_lost_and_found) {
 			char *filename = git_path("lost-found/%s/%s",
 				obj->type == OBJ_COMMIT ? "commit" : "other",
@@ -614,6 +616,7 @@
 static struct option fsck_opts[] = {
 	OPT__VERBOSE(&verbose, "be verbose"),
 	OPT_BOOLEAN(0, "unreachable", &show_unreachable, "show unreachable objects"),
+	OPT_BOOL(0, "dangling", &show_dangling, "show dangling objects"),
 	OPT_BOOLEAN(0, "tags", &show_tags, "report tags"),
 	OPT_BOOLEAN(0, "root", &show_root, "report root nodes"),
 	OPT_BOOLEAN(0, "cache", &keep_cache_objects, "make index objects head nodes"),