RebaseDlg: Edit->Squash sequence incorrectly amended to previous commit

Signed-off-by: Sven Strickroth <email@cs-ware.de>
RebaseDlg: Correctly remember commit message on Edit->Squash sequence

Signed-off-by: Sven Strickroth <email@cs-ware.de>
RebaseDlg: Make Squash->Edit->Squash not to introduce unwanted commit

(fixes issue #2126)

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

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