Change Git passthrough examples to use `refs/heads/*`

What does this MR do?

The way we document Git passthrough chains for some of the groupstatic analysis analysers doesn't actually work. See the commit message for more information.

I've prepared an example project to showcase the behaviour:

• The ruleset config specifies ref = "refs/remotes/origin/main", a perfectly legit branch name for the sast-rules repository.
• The Semgrep pipeline has failed with a misleading error message 'couldn't find remote ref "refs/heads/origin/main"'.

Related issues

• N/A

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