Patches as produced by "git format-patch" have a trailing signature
that is separated from the body by a line with "-- " on it. By default
it contains the Git version. We erroneously include this signature
in the diff we pipe to patch, which fails to apply as a result.
Add a targeted fix to suppress these signatures.
Since :patch transforms its inputs into context-only lines, we can
easily get into a state where a file diff has only context lines.
git apply does not accept a "diff" without any hunk, so let's skip
that.