git-commit --allow-empty

git-commit --allow-empty

It does not usually make sense to record a commit that has the exact
same tree as its sole parent commit and that is why git-commit prevents
you from making such a mistake, but when data from foreign scm is
involved, it is a different story.  We are equipped to represent such an
(perhaps insane, perhaps by mistake, or perhaps done on purpose) empty
change, and it is better to represent it bypassing the safety valve for
native use.

This is primarily for use by foreign scm interface scripts.

Signed-off-by: Junio C Hamano <gitster@pobox.com>