rebase--interactive: don't require what's rebased to be a branch

git rebase allows you to specify a non-branch commit-ish as the "branch"
argument, which leaves HEAD detached when it's finished.  This is
occasionally useful, and this patch brings the same functionality to git
rebase --interactive.

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

git-rebase--interactive.sh

@@ -783,8 +783,6 @@ first and then run 'git rebase --continue' again."
 
 		if test ! -z "$1"
 		then
-			output git show-ref --verify --quiet "refs/heads/$1" ||
-				die "Invalid branchname: $1"
 			output git checkout "$1" ||
 				die "Could not checkout $1"
 		fi