add --path option to git hash-object

add --path option to git hash-object

The --path option allows us to pretend as if the contents being hashed
came from the specified path, and affects which input filter is used via
the attributes mechanism.  This is useful for hashing a temporary file
whose name is different from the path that is meant to have the hashed
contents.

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>