Merge branch 'ak/commit-only-allow-empty' into maint

Merge branch 'ak/commit-only-allow-empty' into maint

"git commit --allow-empty --only" (no pathspec) with dirty index
ought to be an acceptable way to create a new commit that does not
change any paths, but it was forbidden, perhaps because nobody
needed it so far.

* ak/commit-only-allow-empty:
  commit: remove 'Clever' message for --only --amend
  commit: make --only --allow-empty work without paths