Get rid of QRegularExpressions, they are very slow.
Simplify readGitPatch() a lot.
Make reading of the patch about 20 times faster,
especially make readGitDiff() itseft (excluding the calls to
readChunks) working about 1000 times faster for huge diffs.
So, the processing time for e.g. the bottom commit of
qttools module (the import commit) decreased from ~20 seconds
to ~1 second.
Implement nice progress of patch reading.

Change-Id: Ie24786596237bde475e37337663018a8bec086bb
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>