Allow nf-core pipelines download -r to download commits

[muffato]

Supersedes #2906

This is to allow downloading a specific commit of a pipeline, instead of only a branch / tag.
As @ewels suggested in the original pull-request, I only check the commit once we know it's neither a branch nor a tag. It's a single extra query, and it won't download the whole list of possible commits.

I could reuse the tests @fellen31 wrote. Credits to them

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.97%. Comparing base (82eccbe) to head (b0b9559).

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