Checkout fork merge request branch#

Developers at times, need to checkout and build changes from a merge request fork branch. Any one of the following methods can be used:

Checkout COMMIT-SHA of the branch HEAD. It is available on the “Commits” tab of the merge request web page.
```
% git fetch origin <COMMIT-SHA>
% git checkout FETCH_HEAD
```
Checkout branch using the repository URL. The URL with branchname is available as “Source branch” hyperlink on merge request web page.
```
% git fetch <URL> <branchname>
% git checkout FETCH_HEAD
```

Setup a local Git clone to access the merge request branch via the MR-NUMBER. Here use the following in .git/config:

[remote "origin"]
      url = https://gitlab.com/petsc/petsc.git
      pushurl = git@gitlab.com:petsc/petsc.git
      fetch = +refs/heads/*:refs/remotes/origin/*
      fetch = +refs/merge-requests/*:refs/remotes/origin/merge-requests/*

Now, the branch is available to checkout:

% git fetch
% git checkout origin/merge-requests/<MR-NUMBER>/head

Commit and push changes to a merge request fork branch#

Only Owners/Maintainers can push commits to a merge request fork branch. Here, use the ssh-URL for the Git repository.

% git fetch <ssh-URL> <branchname>
% git checkout -b <branchname> FETCH_HEAD
% git push -u <ssh-URL> <branchname>
% (edit/commit)
% git push

Notes:

For example, with merge request at petsc/petsc!8619 we have:

MR-NUMBER = 8619
COMMIT-SHA = 1b741c341f10772b6231f15a4abcef052bfe2d90 (“Copy commit SHA” of the HEAD commit from the “Commits” tab)
URL with branchname = https://gitlab.com/paul.kuehner/petsc/-/tree/add-tao-get-constraints (“Copy link” of “Source branch”)
URL = https://gitlab.com/paul.kuehner/petsc
ssh-URL = git@gitlab.com:paul.kuehner/petsc.git
branchname = add-tao-get-constraints