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