Merge branch 'js/rebase-am'

Instead of going through "git-rebase--am" scriptlet to use the "am" backend, the built-in version of "git rebase" learned to drive the "am" backend directly. * js/rebase-am: built-in rebase: call `git am` directly rebase: teach `reset_head()` to optionally skip the worktree rebase: avoid double reflog entry when switching branches rebase: move `reset_head()` into a better spot