RebaseCommand should ignore submodule modifications
Before a rebase happens the RebaseCommand checks that the working tree
is clean. We don't want to start a rebase on a dirty working tree. If
the working tree is dirty a rebase should not be allowed. But
RebaseCommand should ignore modifications done to submodules. E.g. if a
submodules HEAD points to <x> but the root repository has in index that
the submodule should point to <y> then this should not prohibit a
rebase. Also native git allows a rebase in this case. Since jgit's
StatusCommand has learned to ignore submodule changes this is now used
by the RebaseCommand to determine the repository state correctly.