Merge branch 'jk/stash-options'

Make "git stash something --help" error out, so that users can
safely say "git stash drop --help".

* jk/stash-options:
stash: recognize "--help" for subcommands
stash: complain about unknown flags

diff --cc git-stash.sh
Simple merge

diff --cc t/t3903-stash.sh
Simple merge