git-apply: ignore empty git headers

git-apply: ignore empty git headers

A meaningful (ie non-empty) git patch always has more information in the
header than just the "diff --git" line itself: it needs to have either a
patch associated with it (which implies "---" and "+++" lines in the
header) or it needs to have rename/copy/delete/create information in it.

Just ignore git patches which have no change information. Otherwise we'll
end up with a patch that doesn't have filenames etc filled in, and we'll
be unhappy.