git-apply --binary: clean up and prepare for --reverse

git-apply --binary: clean up and prepare for --reverse

This cleans up the implementation of "git-apply --binary", and
implements reverse application of binary patches (when git-diff
is converted to emit reversible binary patches).

Earlier, the types of encoding (either deflated literal or
deflated delta) were stored in is_binary field in struct patch,
which meant that we cannot store more than one fragment that
differ in the encoding for a patch.  This moves the information
to a field in struct fragment that is otherwise unused for
binary patches, and makes it possible to hang two (or more, but
two is enough) hunks for a binary patch.

The original "binary patch" output from git-diff is internally
parsed into an "is_binary" patch with one fragment.  Upcoming
reversible binary patch output will have two fragments, the
first one being the forward patch and the second one the reverse
patch.

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