redhat: properly handle binary files in patches

Pass --binary to git diff when generating the patch-*-redhat.patch in genspec.sh, so that the patch is applicable also when there are diffs in binary files.

Furthermore, since the patch utility doesn't support applying binary diffs, switch the spec file to use git apply instead of patch. This is OK, since the spec already has BuildRequires: git-core. In fact, this means we can now remove patch from BuildRequires in favor of git-core.