In https://gitlab.com/gitlab-org/gitlab-ce/commit/ebf16ada856efb85424a98848c141f21e609886a
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).