Merge branch 'js/rebase-am' into jch

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

Author: gitster