Merge branch 'mz/rebase'

Merge branch 'mz/rebase'

* mz/rebase: (34 commits)
  rebase: define options in OPTIONS_SPEC
  Makefile: do not install sourced rebase scripts
  rebase: use @{upstream} if no upstream specified
  rebase -i: remove unnecessary state rebase-root
  rebase -i: don't read unused variable preserve_merges
  git-rebase--am: remove unnecessary --3way option
  rebase -m: don't print exit code 2 when merge fails
  rebase -m: remember allow_rerere_autoupdate option
  rebase: remember strategy and strategy options
  rebase: remember verbose option
  rebase: extract code for writing basic state
  rebase: factor out sub command handling
  rebase: make -v a tiny bit more verbose
  rebase -i: align variable names
  rebase: show consistent conflict resolution hint
  rebase: extract am code to new source file
  rebase: extract merge code to new source file
  rebase: remove $branch as synonym for $orig_head
  rebase -i: support --stat
  rebase: factor out call to pre-rebase hook
  ...