checkout: allow detaching to HEAD even when switching to the tip of a branch

You cannot currently checkout the tip of an existing branch
without moving to the branch.

This allows you to detach your HEAD and place it at such a
commit, with:

    $ git checkout master^0

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-checkout.sh

@@ -170,7 +170,7 @@ describe_detached_head () {
 	}
 }
 
-if test -z "$branch$newbranch" && test "$new" != "$old"
+if test -z "$branch$newbranch" && test "$new_name" != "$old_name"
 then
 	detached="$new"
 	if test -n "$oldbranch" && test -z "$quiet"
@@ -180,7 +180,7 @@ If you want to create a new branch from this checkout, you may do so
 (now or later) by using -b with the checkout command again. Example:
   git checkout -b <new_branch_name>"
 	fi
-elif test -z "$oldbranch"
+elif test -z "$oldbranch" && test "$new" != "$old"
 then
 	describe_detached_head 'Previous HEAD position was' "$old"
 fi

t/t7201-co.sh

@@ -3,7 +3,20 @@
 # Copyright (c) 2006 Junio C Hamano
 #
 
-test_description='git-checkout tests.'
+test_description='git-checkout tests.
+
+Creates master, forks renamer and side branches from it.
+Test switching across them.
+
+  ! [master] Initial A one, A two
+   * [renamer] Renamer R one->uno, M two
+    ! [side] Side M one, D two, A three
+  ---
+    + [side] Side M one, D two, A three
+   *  [renamer] Renamer R one->uno, M two
+  +*+ [master] Initial A one, A two
+
+'
 
 . ./test-lib.sh
 
@@ -129,4 +142,52 @@ test_expect_success 'checkout -m with merge conflict' '
 	! test -s current
 '
 
+test_expect_success 'checkout to detach HEAD' '
+
+	git checkout -f renamer && git clean &&
+	git checkout renamer^ &&
+	H=$(git rev-parse --verify HEAD) &&
+	M=$(git show-ref -s --verify refs/heads/master) &&
+	test "z$H" = "z$M" &&
+	if git symbolic-ref HEAD >/dev/null 2>&1
+	then
+		echo "OOPS, HEAD is still symbolic???"
+		false
+	else
+		: happy
+	fi
+'
+
+test_expect_success 'checkout to detach HEAD with branchname^' '
+
+	git checkout -f master && git clean &&
+	git checkout renamer^ &&
+	H=$(git rev-parse --verify HEAD) &&
+	M=$(git show-ref -s --verify refs/heads/master) &&
+	test "z$H" = "z$M" &&
+	if git symbolic-ref HEAD >/dev/null 2>&1
+	then
+		echo "OOPS, HEAD is still symbolic???"
+		false
+	else
+		: happy
+	fi
+'
+
+test_expect_success 'checkout to detach HEAD with HEAD^0' '
+
+	git checkout -f master && git clean &&
+	git checkout HEAD^0 &&
+	H=$(git rev-parse --verify HEAD) &&
+	M=$(git show-ref -s --verify refs/heads/master) &&
+	test "z$H" = "z$M" &&
+	if git symbolic-ref HEAD >/dev/null 2>&1
+	then
+		echo "OOPS, HEAD is still symbolic???"
+		false
+	else
+		: happy
+	fi
+'
+
 test_done