Merge branch 'jc/merge-detached-head-name'

Merge branch 'jc/merge-detached-head-name'

The default merge message prepared by "git merge" records the name
of the current branch; the name can be overridden with a new option
to allow users to pretend a merge is made on a different branch.

* jc/merge-detached-head-name:
  merge: allow to pretend a merge is made into a different branch