So we can distinguish between the permissions on the source and the
target project.
- `create_merge_request_from` indicates a user can create a merge
request with the project as a source_project
- `create_merge_request_in` indicates a user can create a merge
request with the project as a target_project
This ensures that we have more visibility in the number of SQL queries
that are executed in web requests. The current threshold is hardcoded to
100 as we will rarely (maybe once or twice) change it.
In production and development we use Sentry if enabled, in the test
environment we raise an error. This feature is also only enabled in
production/staging when running on GitLab.com as it's not very useful to
other users.
If a merge request was created with a branch name that also matched a tag name,
we'd generate a comparison to or from the tag respectively, rather than the
branch. Merging would still use the branch, of course.
To avoid this, ensure that when we get the branch heads, we prepend the
reference prefix for branches, which will ensure that we generate the correct
comparison.
also, I refactored the MergeRequest#fetch_ref method to express
the side-effect that this method has.
MergeRequest#fetch_ref -> MergeRequest#fetch_ref!
Repository#fetch_source_branch -> Repository#fetch_source_branch!
