Unified diff parsing: maintain in-header boolean state to determine whether to drop from-file/to-file lines.

The relevant diffutils Unified Format documentation is quite clear that there are exactly two header lines in each unified format diff file.

The JSON format that we read from in diffoscope often (always?) already omits these lines, but there is code to check for their presence.

I don't feel entirely comfortable removing the code that filters the header-lines, in case that is used for some existing backwards-compatibility reason. So instead, track an initially-true boolean flag that we disable when we encounter the first hunk marker.

Not elegant, but should solve the problem.

Resolves #372 (closed).