Checkout remote tag if checkout remote branch fails

[bentsherman]

Close #4246

checkoutRemoteBranch() now falls back to checking for remote tag if the branch checkout fails (which it will if the revision is a tag).

For some reason though, the returned ref is null, even though a tag is considered a ref, so the message will say "checkout at null: revision [abcdef]". Not sure how to deal with that.

To test:

./launch.sh drop epi2me-labs/wf-basecalling
./launch.sh pull -r v1.0.0 epi2me-labs/wf-basecalling

cd $HOME/.nextflow/assets/epi2me-labs/wf-basecalling
git tag -d v1.0.1
cd -

./launch.sh pull -r v1.0.1 epi2me-labs/wf-basecalling

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[pditommaso]

Any chance to add a unit test?