dim: Try git-merge --ff-only before git-rebase -i when updating branches.

When a branch can be fast-forwarded, try it first before rebasing.
This will prevent a whole lot of editor windows opening with 'noop'
when running dim ub.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>