devel-docs: add some more tricks to review MR in Gitlab.

- The trick to fetch automatically all MRs in read-only branches.
- How to temporarily checkout and push to a contributor's remote without
  actually hard-naming their remote repo and polluting your local repo
  with every new MR.

devel-docs/gitlab-mr.md

@@ -44,3 +44,88 @@ the pipeline. Once the pipeline ends, the flatpak can be installed by:
 - Locally run: `flatpak install --user ./gimp-git.flatpak`
   It should propose you to install the flatpak, allowing you to test.
 - After testing, you can uninstall with: `flatpak remove org.gimp.GIMP//master`
+
+## Reviewing MR branches
+
+Reviewing merge requests on the Gitlab interface often leads to poor
+review, because:
+
+- It doesn't show tabs, trailing whitespaces and other space issues
+  which a well-configured CLI git would usually emphasize with colors.
+- The commit history is not emphasized, only the final results, but it's
+  very important to check individual commits, as well as usable commit
+  messages.
+- It's anyway usually much easier to review patches on your usual
+  workflow environment rather than in a hard-to-use web interface.
+
+There are ways to work on your local environments.
+
+### Fetching MR branches automatically (read-only)
+
+This first one is more of a trick, but an incredibly useful one.
+Unfortunately it is read-only, so it means you can review but not edit
+the MR yourself. Nevertheless since having to edit a MR should be the
+exception, not the rule, it's actually not too bad.
+
+Edit your `.git/config` by adding a second "fetch =" rule to the
+"origin" remote. It should read:
+
+```
+[remote "origin"]
+	fetch = +refs/heads/*:refs/remotes/origin/*
+	fetch = +refs/merge-requests/*/head:refs/remotes/origin/merge-requests/*
+	url = git@gitlab.gnome.org:GNOME/gimp.git
+```
+
+From now on, when you `git pull` or `git fetch` the origin remote, any
+new or updated merge request will also be fetched. \o/
+
+### Pushing to a third-party MR branch
+
+There are cases when you want to push to the MR branch. It should stay
+rare occasions, but it can be for instance when the contributor seems
+stuck and doesn't know how to do some things; or maybe one doesn't
+understand instructions; sometimes also some contributors disappear
+after pushing their patch and never answer to review anymore.
+
+When this happen, you could merge the commit and fix it immediately
+after (but it's never good to leave the repo in a bad state, even for
+just a few minutes). You could also apply, fix and push the fixed
+commits directly, but then the MR has to be closed and it doesn't look
+like it was applied (which is not the end of the world, but it's still
+nicer to show proper status on which patches were accepted or not).
+Moreover you would not be able to pass the CI build.
+
+So we will fetch the remote yet without naming the remote:
+
+- Click the "Check out branch" button below the merge request
+  description. Gitlab gives you instructions but we will only use the
+  first step ("Fetch and check out the branch for this merge request").
+  For instance if contributor `xyz` created the branch `fix-bug-123` on
+  their own remote, you would run:
+
+```
+git fetch "git@gitlab.gnome.org:xyz/gimp.git" 'fix-bug-123'
+git checkout -b 'xyz/fix-bug-123' FETCH_HEAD
+```
+
+- Now that you are in a local branch with their code, make your fix, add
+  a local commit.
+
+- Finally push to the contributor's own remote with the call:
+
+```
+git push git@gitlab.gnome.org:xyz/gimp.git xyz/fix-bug-123:fix-bug-123
+```
+
+  This assumes that the contributor checked the option "*Allow commits
+  from members who can merge to the target branch.*" (which we ask
+  contributors to check, and it's set by default)
+
+- Now check the MR page. It will normally be updated with your new
+  commit(s) and a new pipeline should be triggered.
+
+- Finally if you don't need the local branch anymore, you may delete it
+  locally. The nice thing is that since you didn't name the remote, it
+  doesn't pollute your git output and all data will be simply disposed
+  of the next time `git gc` runs (implicitly or explicitly).