[PATCH v2 5/6] clone: allow --branch to take a tag

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

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>
---
 Although 'git clone --depth 1 --branch <tag>' still needs fixups on
 top. I'll do that later.

 Documentation/git-clone.txt |    5 +++--
 builtin/clone.c             |   14 ++++++++++++++
 t/t5601-clone.sh            |    9 +++++++++
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-clone.txt b/Documentation/git-clone.txt
index 4b8b26b..db2b29c 100644
--- a/Documentation/git-clone.txt
+++ b/Documentation/git-clone.txt
@@ -146,8 +146,9 @@ 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>::
diff --git a/builtin/clone.c b/builtin/clone.c
index 914fd6b..0d4b9ab 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -734,6 +734,14 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 			strbuf_release(&head);
 
 			if (!our_head_points_at) {
+				strbuf_addstr(&head, "refs/tags/");
+				strbuf_addstr(&head, option_branch);
+				our_head_points_at =
+					find_ref_by_name(mapped_refs, head.buf);
+				strbuf_release(&head);
+			}
+
+			if (!our_head_points_at) {
 				warning(_("Remote branch %s not found in "
 					"upstream %s, using HEAD instead"),
 					option_branch, option_origin);
@@ -776,6 +784,12 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 			install_branch_config(0, head, option_origin,
 					      our_head_points_at->name);
 		}
+	} else if (our_head_points_at) {
+		const struct ref *ref = our_head_points_at;
+		struct commit *c = lookup_commit_reference(ref->old_sha1);
+		/* Source had detached HEAD pointing somewhere. */
+		update_ref(reflog_msg.buf, "HEAD", c->object.sha1,
+			   NULL, REF_NODEREF, DIE_ON_ERR);
 	} else if (remote_head) {
 		/* Source had detached HEAD pointing somewhere. */
 		update_ref(reflog_msg.buf, "HEAD", remote_head->old_sha1,
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index e0b8db6..67869b4 100755
--- a/t/t5601-clone.sh
+++ b/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
-- 
1.7.8.36.g69ee2