Do not allow non-ff-rebase if there are uncommitted changes

Do not allow non-ff-rebase if there are uncommitted changes

With this change jgit checks for uncommitted changes before a rebase is
started. This is also done by native git. One reason is that an abort
would override such changes. The check is skipped for a non-interactive
rebase when it will result in a fast-forward. In this case there can be
only checkout conflicts but no merge conflicts, so there cannot be an
abort which overrides uncommitted changes.

Bug: 422352
Change-Id: I1e0b59b2a4d80a686b67a6729e441924362b1236
Signed-off-by: Stefan Lay <stefan.lay@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>