Merge branch 'mj/pull-rebase-autostash'

"git pull --rebase" learned "--[no-]autostash" option, so that
the rebase.autostash configuration variable set to true can be
overridden from the command line.

* mj/pull-rebase-autostash:
  t5520: test --[no-]autostash with pull.rebase=true
  t5520: reduce commom lines of code
  t5520: factor out common "failing autostash" code
  t5520: factor out common "successful autostash" code
  t5520: use better test to check stderr output
  t5520: ensure consistent test conditions
  t5520: use consistent capitalization in test titles
  pull --rebase: add --[no-]autostash flag
  git-pull.c: introduce git_pull_config()