TortoiseGitMerge: Fixed possible crash while applying a patch

https://www.crash-server.com/Problem.aspx?ClientID=TortoiseGit&ProblemID=22925
https://www.crash-server.com/Problem.aspx?ClientID=TortoiseGit&ProblemID=20793

Signed-off-by: Sven Strickroth <email@cs-ware.de>
TortoiseGitMerge: When merging, left/right views should not set modified when using text block from both sides

Based on TortoiseSVN revision 24270

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>