Merge branch 'ah/rebase-merges-config'

Streamline --rebase-merges command line option handling and
introduce rebase.merges configuration variable.

* ah/rebase-merges-config:
  rebase: add a config option for --rebase-merges
  rebase: deprecate --rebase-merges=""
  rebase: add documentation and test for --no-rebase-merges

diff --cc builtin/rebase.c
Simple merge