mbox: recognize patch consisting entirely of GIT binary diffs
mbox: recognize patch consisting entirely of GIT binary diffs

A patch consisting entirely of binary diffs is not recognized by
Patch. Make Mbox.is_patch detect binary diffs to fix it.

Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>

pvital/patchew,pvital/patchew,pvital/patchew,pvital/patchew