Guard access to LogDlg revision file list

Signed-off-by: Sven Strickroth <email@cs-ware.de>
TortoiseGitMerge: Check for an invalid index and bail out if it's invalid

Fixes Crash in TortoiseMerge trying to find moved blocks

Based on TortoiseSVN revision 24670

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