Merge branch 'vv/merge-squash-with-explicit-commit' into maint

Merge branch 'vv/merge-squash-with-explicit-commit' into maint

"git merge --squash" is designed to update the working tree and the
index without creating the commit, and this cannot be countermanded
by adding the "--commit" option; the command now refuses to work
when both options are given.

* vv/merge-squash-with-explicit-commit:
  merge: refuse --commit with --squash