1.7.6

Fix sc move, sc restack and other restacking commands crashing when a branch
they need to rebase is checked out in another worktree. Shortcake now rebases it
on a detached HEAD and moves the branch from inside its worktree, so that
worktree's files follow. It refuses with a clear error when that worktree has
uncommitted changes or an operation in progress, and sc continue / sc abort
keep the worktree in sync after a conflict.

Also fix sc move crashing with a KeyError when the branch to move doesn't
exist; it now reports that the branch wasn't found.

This release was contributed by @patrick91 in #151