git-blame: no rev means start from the working tree file.

git-blame: no rev means start from the working tree file.

Warning: this changes the semantics.

This makes "git blame" without any positive rev to start digging
from the working tree copy, which is made into a fake commit
whose sole parent is the HEAD.

It also adds --contents <file> option to pretend as if the
working tree copy has the contents of the named file.  You can
use '-' to make the command read from the standard input.

If you want the command to start annotating from the HEAD
commit, you need to explicitly give HEAD parameter.

Signed-off-by: Junio C Hamano <junkio@cox.net>