tests: Add a test for sync with potential conflicts

[carlosfg-flu]

This is an scenario where GUW sync command raises some git conflicts that
are avoidable:

* 0632076 (HEAD -> A) foo
| * b1ac2a0 (B) bar
| * fd1dd3f foo
|/
* 5ccd4f9 (main, final) init

In this scenario the branch A was rebased, hypothetically because a
reviewed asked for a change. Then, when we try to "sync" with GUW, it
launches the command:

git rebase origin/A B --onto=final

The range "origin/A..B" is wrong, the correct one should be "B~..B" because
it includes only the commits within the "B" branch. This command
doesn't raises conflicts and gives the expected result:

git rebase B~ B --onto=final

[carlosfg-flu]

When a feature, let's say feature1, enters into reviewing mode due to a merge request,
feature1 will not be sent upstream but a new branch called feature1-reviewing.

gitGraph TB:
  commit
  commit
  branch feature1
  commit

Loading

So this scenario won't be possible if the workflow is followed correctly.

PS: We might want to use this way of testing, with a locally generated git repo, instead of polluting the GUW official repository with testing inmortal branches.