diff: improved handling of Git diffs with " quoting

Kallithea would intentionally and explicitly fail with an ugly exception when
trying to parse Git diffs with quoted filenames.

Improve this by parsing quotes ... and ignore them, as long as they are
matching. The content inside the quotes might be \-escaped ... but that could
potentially also be the case without quoting. We will fix that later.

Adding some test cases that before would have failed to parse and raised an
exception.

Thanks to stypr of Flatt Security for raising this.