DiffEditor: Parse git diffs properly

Do not put filenames into a regular expression and then hope for
the best... any file with any character in its name that has a
special meaning for a regexp will break the parsing.

Task-number: QTCREATORBUG-14322
Change-Id: Ifab513d675168aff041f20d2b3f06a8d27919aa7
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>