Process cards between new_sha and old_sha

Previously it processed only the newest commit, not going deeper (as I
did not understand what those really are). This fixes the critical but,
but leaves at least two problems open:

  - If a push contains a merge commit, it might move/comment cards
again. Solution: use rebase rather then merge or merge/push carefully.
  - Should figure out what happens when tags or new branches are pushed.

Fixes #9

Signed-off-by: zmilojko <zeljko@zwr.fi>