apply: allow-binary-replacement.

apply: allow-binary-replacement.

A new option, --allow-binary-replacement, is introduced.

When you feed a diff that records full SHA1 name of pre- and
post-image blob on its index line to git-apply with this option,
the post-image blob replaces the path if what you have in the
working tree matches the pre-image _and_ post-image blob is
already available in the object directory.

Later we _might_ want to enhance the diff output to also include
the full binary data of the post-image, to make this more
useful, but this is good enough for local rebasing application.

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