Update the cla-check

[james-bruten-mo]

Merging the changes from #50 into main having been verified on the develop branch in the git playground

[james-bruten-mo]

Hi @yaswant this one is now ready for review.
It uses the github pr merge ref to check for modification of the file in a PR when the cla has already been signed. We also need to check that this ref exists - it won't if there is a merge conflict. In that case we allow it to pass as the CI will rerun once the conflict has been fixed and it will be checked there instead.

I've done a couple of PRs in git_playground to demonstrate:

• MetOffice/git_playground#114 is created from the stable branch and includes a merge conflict. I've added my name to the cla here - the history of the runs without my addition should be visible.
• MetOffice/git_playground#115 is created from main (having added my name to the main CONTRIBUTORS). I've then edited the file in the branch to trigger the modified error.

[yaswant]

Looks fine James. Just a minor suggestion to reduce network calls.

[yaswant]

MetOffice/git_playground#115

the cla-modified label doesnot show correct background colour - should be orange I think.

[james-bruten-mo]

It looks like the colour and description entries in the label api calls were the wrong way round. Works as expected now

[yaswant]

@james-bruten-mo I have a strong feeling that we can shorten this workflow, perhaps in a separate PR. However, based on your tests I am happy to approve this for main.