cherry-pick: add --allow-empty-message option

cherry-pick: add --allow-empty-message option

Scripts such as "git rebase -i" cannot currently cherry-pick commits
which have an empty commit message, as git cherry-pick calls git
commit without the --allow-empty-message option.

Add an --allow-empty-message option to git cherry-pick which is passed
through to git commit, so this behaviour can be overridden.

Signed-off-by: Chris Webb <redacted>
Reviewed-by: Neil Horman <redacted>
Signed-off-by: Junio C Hamano <redacted>