dim: abstract helper for listing unmerged tags

Make the pull request code a bit easier to grasp. Use git log pretty
format for easier extraction of the tags.

v2: Only replace newlines with space, add comment about it (Daniel)

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>