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

"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

Documentation/merge-options.txt

@@ -102,6 +102,8 @@ merge.
 +
 With --no-squash perform the merge and commit the result. This
 option can be used to override --squash.
++
+With --squash, --commit is not allowed, and will fail.
 
 -s <strategy>::
 --strategy=<strategy>::