Prevent possible null dereferences

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Initialize member variables

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Drop ColumnProvider support

Columns were not supported by TortoiseGit so far and Microsoft dropped support for this after WinXP. Hence, we're not going to need this any more.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
TortoiseGitBlame: Show more icons in left pane context menu

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