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