Merge branch 'jc/merge-compact-summary' into next

"git merge/pull" has been taught the "--compact-summary" option to
use the compact-summary format, intead of diffstat, when showing
the summary of the incoming changes.

* jc/merge-compact-summary:
  merge/pull: extend merge.stat configuration variable to cover --compact-summary
  merge/pull: add the "--compact-summary" option