Revert "patch: fix pch_context() for unified diffs with no leading context"

This reverts commit f97b6a8f84b3ed209c2aea0958a7b889d0bf27ed, as it
turns out our fuzz implementation is just too naive. We can have more
leading context than trailing context and vice-versa, so we can't really
assume they're the same.

Restore the previous bug and we can work on it post-branch.

Reported by: cy