How to merge as part of the pipeline?

[Mathew Vaughan]

What I'm attempting to do:

When a change is pushed on a feature branch that matches the pattern 'merge**' I would like the tests to be run and for the change to be merged into our 'develop' branch.

I attempt to checkout develop so that I can merge, however I receive this error:

error: pathspec 'develop' did not match any file(s) known to git.

I tried running git fetch and git branch, but the only branch available is the feature branch. I believe this may be down to the fact the Build Setup clones a specific branch.

How can I access my develop branch as part of my build pipeline?