In ebf16ada85
we introduced a SHA validator, to ensure that the data provided in
merge request diffs, was legit. Nevertheless, the validator
assumed that the SHA should be 40 chars long.
When we import a project from BitBucket, the retrieved SHA is
shorter (12 chars long). Therefore, this validator prevented to
create a valid MergeRequestDiff for ever MergeRequest (triggering
an exception).
