Allow git-checkout when on a non-existant branch.

I've seen some users get into situtations where their HEAD
symbolic-ref is pointing at a non-existant ref.  (Sometimes this
happens during clone when the remote repository lacks a 'master'
branch.)  If this happens the user is unable to use git-checkout
to switch branches as there is no prior commit to merge from.

So instead of giving the user low-level errors about how HEAD
can't be resolved and how not a single revision was given change
the type of checkout to be a force and go through with the user's
request anyway.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-checkout.sh

@@ -4,8 +4,8 @@ USAGE='[-f] [-b <new_branch>] [-m] [<branch>] [<paths>...]'
 SUBDIRECTORY_OK=Sometimes
 . git-sh-setup
 
-old=$(git-rev-parse HEAD)
 old_name=HEAD
+old=$(git-rev-parse --verify $old_name 2>/dev/null)
 new=
 new_name=
 force=
@@ -139,6 +139,13 @@ fi
 	die "git checkout: to checkout the requested commit you need to specify 
               a name for a new branch which is created and switched to"
 
+if [ "X$old" = X ]
+then
+	echo "warning: You do not appear to currently be on a branch." >&2
+	echo "warning: Forcing checkout of $new_name." >&2
+	force=1
+fi
+
 if [ "$force" ]
 then
     git-read-tree --reset -u $new

t/t7201-co.sh

@@ -31,6 +31,15 @@ test_expect_success setup '
 	git checkout master
 '
 
+test_expect_success "checkout from non-existing branch" '
+
+	git checkout -b delete-me master &&
+	rm .git/refs/heads/delete-me &&
+	test refs/heads/delete-me = "$(git symbolic-ref HEAD)" &&
+	git checkout master &&
+	test refs/heads/master = "$(git symbolic-ref HEAD)"
+'
+
 test_expect_success "checkout with dirty tree without -m" '
 
 	fill 0 1 2 3 4 5 >one &&