patch-id: fix stable patch id for binary / header-only

patch-id: fix stable patch id for binary / header-only

Patch-ids for binary patches are found by hashing the object
ids of the before and after objects in succession. However in
the --stable case, there is a bug where hunks are not flushed
for binary and header-only patch ids, which would always result
in a patch-id of 0000. The --unstable case is currently correct.

Reorder the logic to branch into 3 cases for populating the
patch body: header-only which populates nothing, binary which
populates the object ids, and normal which populates the text
diff. All branches will end up flushing the hunk.

Don't populate the ---a/ and +++b/ lines for binary diffs, to correspond
to those lines not being present in the "git diff" text output.
This is necessary because we advertise that the patch-id calculated
internally and used in format-patch is the same that what the
builtin "git patch-id" would produce when piped from a diff.

Update the test to run on both binary and normal files.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>