Let "git fast-import -h" (with no other arguments) print usage
before exiting, even when run outside any repository.

Cc: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>