Allow forcing of a parent commit, even if the parent is not a direct one.

Allow forcing of a parent commit, even if the parent is not a direct one.

This can be used to compress multiple changesets into one, for example
like

	git cvsexportcommit -P cvshead mybranch

without having to do so in git first.

Signed-off-by: Simon 'corecode' Schubert <redacted>
Signed-off-by: Junio C Hamano <redacted>