MergeRequest#create_merge_request_diff and MergeRequest#reload_diff are
the only places where we generate a new MR diff so that's where we
should fetch the ref.

This also ensures that the ref is not fetched when we call
merge_request.merge_request_diffs.create in
Github::Import#fetch_pull_requests.
Signed-off-by: Rémy Coutable <remy@rymai.me>

The benefit is that we don't even have to create temp source/target
branches.

Also, when the source branch of the imported MR is from a fork, name it
"user:branch" to mimic how we display it for MR when forks actually
exists.
Signed-off-by: Rémy Coutable <remy@rymai.me>

We don't need to check if the existing branch contains a specific SHA
since we only use the branch's name, the SHA is taken from the pull
request raw `sha` data.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Also, improved a bit the method names in
Github::Representation::PullRequest.

Last but not least, ensure temp branch names doesn't contain a `/` as
this would create the ref in a subfolder in `refs/heads` (e.g.
`refs/heads/gh-123/456/rymai/foo`), and would leave empty directories
upon branch deletion (e.g. `refs/heads/gh-123/456/rymai/`.
Signed-off-by: Rémy Coutable <remy@rymai.me>

update MR diff

[ci skip]

Merge branch '37111-monitoring-graph-empty-states-are-gigantic-on-screen-sm-viewport-sizes' into 'master'

Resolve "Monitoring graph empty states are gigantic on `screen-sm` viewport sizes"

Closes #37111

See merge request gitlab-org/gitlab-ce!14259

Clarify where the artifacts metadata .gz is generated

See merge request gitlab-org/gitlab-ce!14695

Fix inconsistent use of sentence case

See merge request gitlab-org/gitlab-ce!14668

Fix username and ID not logging in production_json.log for Git activity

Closes gitlab-ee#3611

See merge request gitlab-org/gitlab-ce!14647

improve merge request widget status icon UX

Closes #37229

See merge request gitlab-org/gitlab-ce!14200

[ci skip]

Cleanup Select2 dropdown styles

Closes #37279

See merge request gitlab-org/gitlab-ce!14557

Makes `@mentions links` have a different styling for better separation

See merge request gitlab-org/gitlab-ce!14377

Devise sets `current_user`, but not all controllers authenticate users by
session tokens. Try to use the controller-defined `authenticated_user` if
`current_user` is not available.

Closes gitlab-org/gitlab-ee#3611

Update unf_ext from 0.0.7.2 to 0.0.7.4 (#38368)

Closes #38368

See merge request gitlab-org/gitlab-ce!14601

Add link to OpenID Connect documentation

See merge request gitlab-org/gitlab-ce!14368

Adjusts tag link to avoid underlining spaces

Closes #38122

See merge request gitlab-org/gitlab-ce!14544

Sidekiq::MemoryKiller: Add missing space in log message

See merge request gitlab-org/gitlab-ce!14553

explain target_project_id vs project_id

Closes #38539

See merge request gitlab-org/gitlab-ce!14594

Fix edit project service cancel button position

See merge request gitlab-org/gitlab-ce!14596

Cleanup the fork layout

See merge request gitlab-org/gitlab-ce!14617

Fix the default branches sorting to actually be 'Last updated'

Closes #26890

See merge request gitlab-org/gitlab-ce!14295

Update images

Closes #38487

See merge request gitlab-org/gitlab-ce!14655

Include RevList error messages in exceptions

See merge request gitlab-org/gitlab-ce!14658

Let fetch_ref pull from Gitaly instead of from disk

Closes gitaly#585

See merge request gitlab-org/gitlab-ce!14588

Add missing exception variable

See merge request gitlab-org/gitlab-ce!14677

Use a UNION ALL for getting merge request notes

Closes #38508

See merge request gitlab-org/gitlab-ce!14620

Load only the currently needed JS locale file

Closes #38470

See merge request gitlab-org/gitlab-ce!14533

Fix AddFastForwardOptionToProject migration

See merge request gitlab-org/gitlab-ce!14676