Add Squash/Fixup support for rebase interactive in RebaseCommand

Add Squash/Fixup support for rebase interactive in RebaseCommand

The rebase command now supports squash and fixup. Both actions are not
allowed as the first step of the rebase.

In JGit, before any rebase step is performed, the next commit is
already cherry-picked. This commit keeps that behaviour. In case of
squash or fixup a soft reset to the parent is perfomed afterwards.

CQ: 7684
Bug: 396510
Change-Id: I3c4190940b4d7f19860e223d647fc78705e57203
Signed-off-by: Tobias Pfeifer <to.pfeifer@web.de>
Signed-off-by: Stefan Lay <stefan.lay@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>