clone: allow --branch to take a tag

Because a tag ref cannot be put to HEAD, HEAD will become detached.
This is consistent with "git checkout <tag>".

This is mostly useful in shallow clone, where it allows you to clone a
tag in addtion to branches.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -147,8 +147,9 @@ objects from the source repository into a pack in the cloned repository.
 -b <name>::
 	Instead of pointing the newly created HEAD to the branch pointed
 	to by the cloned repository's HEAD, point to `<name>` branch
-	instead. In a non-bare repository, this is the branch that will
-	be checked out.
+	instead. `--branch` can also take tags and treat them like
+	detached HEAD. In a non-bare repository, this is the branch
+	that will be checked out.
 
 --upload-pack <upload-pack>::
 -u <upload-pack>::

builtin/clone.c

@@ -420,6 +420,15 @@ static struct ref *find_remote_branch(const struct ref *refs, const char *branch
 	strbuf_addstr(&head, branch);
 	ref = find_ref_by_name(refs, head.buf);
 	strbuf_release(&head);
+
+	if (ref)
+		return ref;
+
+	strbuf_addstr(&head, "refs/tags/");
+	strbuf_addstr(&head, branch);
+	ref = find_ref_by_name(refs, head.buf);
+	strbuf_release(&head);
+
 	return ref;
 }
 
@@ -441,8 +450,12 @@ static struct ref *wanted_peer_refs(const struct ref *refs,
 		if (!remote_head && option_branch)
 			warning(_("Could not find remote branch %s to clone."),
 				option_branch);
-		else
+		else {
 			get_fetch_map(remote_head, refspec, &tail, 0);
+
+			/* if --branch=tag, pull the requested tag explicitly */
+			get_fetch_map(remote_head, tag_refspec, &tail, 0);
+		}
 	} else
 		get_fetch_map(refs, refspec, &tail, 0);
 
@@ -515,6 +528,11 @@ static void update_head(const struct ref *our, const struct ref *remote,
 			update_ref(msg, "HEAD", our->old_sha1, NULL, 0, DIE_ON_ERR);
 			install_branch_config(0, head, option_origin, our->name);
 		}
+	} else if (our) {
+		struct commit *c = lookup_commit_reference(our->old_sha1);
+		/* --branch specifies a non-branch (i.e. tags), detach HEAD */
+		update_ref(msg, "HEAD", c->object.sha1,
+			   NULL, REF_NODEREF, DIE_ON_ERR);
 	} else if (remote) {
 		/*
 		 * We know remote HEAD points to a non-branch, or

t/t5500-fetch-pack.sh

@@ -311,4 +311,19 @@ EOF
 	test_cmp count6.expected count6.actual
 '
 
+test_expect_success 'shallow cloning single tag' '
+	git clone --depth 1 --branch=TAGB1 "file://$(pwd)/." shallow7 &&
+	cat >taglist.expected <<\EOF &&
+TAGB1
+TAGB2
+EOF
+	GIT_DIR=shallow7/.git git tag -l >taglist.actual &&
+	test_cmp taglist.expected taglist.actual &&
+
+	echo "in-pack: 7" > count7.expected &&
+	GIT_DIR=shallow7/.git git count-objects -v |
+		grep "^in-pack" > count7.actual &&
+	test_cmp count7.expected count7.actual
+'
+
 test_done

t/t5601-clone.sh

@@ -271,4 +271,13 @@ test_expect_success 'clone from original with relative alternate' '
 	grep /src/\\.git/objects target-10/objects/info/alternates
 '
 
+test_expect_success 'clone checking out a tag' '
+	git clone --branch=some-tag src dst.tag &&
+	GIT_DIR=src/.git git rev-parse some-tag >expected &&
+	test_cmp expected dst.tag/.git/HEAD &&
+	GIT_DIR=dst.tag/.git git config remote.origin.fetch >fetch.actual &&
+	echo "+refs/heads/*:refs/remotes/origin/*" >fetch.expected &&
+	test_cmp fetch.expected fetch.actual
+'
+
 test_done