Fix searching for repeated characters in files.

Searching for aaaa in aaaaaaaa would give 5 results instead of 2 in case
of non-regexp search.

Task-number: QTCREATORBUG-7883
Change-Id: I5936090b3eef5e68405201fe63d02bf0dd2469d5
Reviewed-by: David Schulz <david.schulz@digia.com>