Detect when 'new changes' are not in fact changes

When an MR is rebased, merges will be blocked with 'new changes were added'. In many cases rebases do not add new changes, and the diff will not have changed.

We should detect when rebases do not affect the diff, and thus should not block merge or cause approvals to be removed.

Proposal

When we calculate diffs, hash them to some SHA and store that. When the MR is updated, compare the two diff-hashes (old and new) and if they are identical, do not mark the MR as updated.