RebaseDlg: Fix incorrect warning on unclean working tree on Squash(Conflict)->Squash sequence

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

(fixes issue #2126)

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