get_sha1(): :path and :[0-3]:path to extract from index.

Earlier patch to say <ent>:<path> by Linus was very useful, and
this extends the same idea to the current index.  An sha1
expression :<path> extracts the object name for the named path
from the current index.

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