Fix margins of submodule diff dialog

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Hide "Show unmodified files"

In order to not confuse users about unimplemented features.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Drop unused Git history dialog

Don't know why it was added, maybe the ancestor of the log dialog.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Do not enable spell check in Rebase command line text box

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