peel_onion(): teach $foo^{object} peeler

A string that names an object can be suffixed with ^{type} peeler to
say "I have this object name; peel it until you get this type. If
you cannot do so, it is an error".  v1.8.2^{commit} asks for a commit
that is pointed at an annotated tag v1.8.2; v1.8.2^{tree} unwraps it
further to the top-level tree object.  A special suffix ^{} (i.e. no
type specified) means "I do not care what it unwraps to; just peel
annotated tag until you get something that is not a tag".

When you have a random user-supplied string, you can turn it to a
bare 40-hex object name, and cause it to error out if such an object
does not exist, with:

	git rev-parse --verify "$userstring^{}"

for most objects, but this does not yield the tag object name when
$userstring refers to an annotated tag.

Introduce a new suffix, ^{object}, that only makes sure the given
name refers to an existing object.  Then

	git rev-parse --verify "$userstring^{object}"

becomes a way to make sure $userstring refers to an existing object.

This is necessary because the plumbing "rev-parse --verify" is only
about "make sure the argument is something we can feed to get_sha1()
and turn it into a raw 20-byte object name SHA-1" and is not about
"make sure that 20-byte object name SHA-1 refers to an object that
exists in our object store".  When the given $userstring is already
a 40-hex, by definition "rev-parse --verify $userstring" can turn it
into a raw 20-byte object name.  With "$userstring^{object}", we can
make sure that the 40-hex string names an object that exists in our
object store before "--verify" kicks in.

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

Documentation/revisions.txt

@@ -116,6 +116,11 @@ some output processing may assume ref names in UTF-8.
   object of that type is found or the object cannot be
   dereferenced anymore (in which case, barf).  '<rev>{caret}0'
   is a short-hand for '<rev>{caret}\{commit\}'.
++
+'rev{caret}\{object\}' can be used to make sure 'rev' names an
+object that exists, without requiring 'rev' to be a tag, and
+without dereferencing 'rev'; because a tag is already an object,
+it does not have to be dereferenced even once to get to an object.
 
 '<rev>{caret}\{\}', e.g. 'v0.99.8{caret}\{\}'::
   A suffix '{caret}' followed by an empty brace pair

sha1_name.c

@@ -594,7 +594,7 @@ struct object *peel_to_type(const char *name, int namelen,
 	while (1) {
 		if (!o || (!o->parsed && !parse_object(o->sha1)))
 			return NULL;
-		if (o->type == expected_type)
+		if (expected_type == OBJ_ANY || o->type == expected_type)
 			return o;
 		if (o->type == OBJ_TAG)
 			o = ((struct tag*) o)->tagged;
@@ -645,6 +645,8 @@ static int peel_onion(const char *name, int len, unsigned char *sha1)
 		expected_type = OBJ_TREE;
 	else if (!strncmp(blob_type, sp, 4) && sp[4] == '}')
 		expected_type = OBJ_BLOB;
+	else if (!prefixcmp(sp, "object}"))
+		expected_type = OBJ_ANY;
 	else if (sp[0] == '}')
 		expected_type = OBJ_NONE;
 	else if (sp[0] == '/')