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

"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

diff --cc Documentation/git-commit.txt
Simple merge

diff --cc builtin/commit.c
Simple merge

diff --cc t/t7501-commit.sh
Simple merge