rebase: refuse to switch to branch already checked out elsewhere
The invocation "git rebase <upstream> <branch>" switches to <branch>
before performing the rebase operation. However, unlike git-switch,
git-checkout, and git-worktree which all refuse to switch to a branch
that is already checked out in some other worktree, git-rebase switches
to <branch> unconditionally. Curb this careless behavior by making
git-rebase also refuse to switch to a branch checked out elsewhere.
Reported-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>