Gitlab: "unable to determine merge request ID"

Created by: lguychard

From Sentry: https://sentry.io/organizations/sourcegraph/issues/801327036/?project=1334031&query=is%3Aunresolved+extension_version%3A19.10.9.1226+extension_version%3A19.10.9.1226&statsPeriod=14d

It seems like this is due to expecting the URL to match merge_requests/(.*)/diffs in getDiffPageInfo() even though getPageInfo() (called by resolveView()) matches any page where the URL includes ${owner}/${projectName}/merge_requests