TortoiseGitMerge: Do not prompt unsaved edit if view is hidden

Based on my patch for TortoiseSVN revision 24367

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
TortoiseGitMerge: Improve space/tab inconsistency detection

* only if there are consecutive spaces to match a tab is the file marked as using spaces
* only if there are tabs is the file marked as using tabs
* only if both tabs and spaces are used is the file marked as space/tab inconsistent

Based on TortoiseSVN revision 24370

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