Add a check to warn if you have different branches checked out when building cross-repo docs

[debadair]

If you don't have the same branch checked out in each repo, you can run into confusing errors when building docs that pull content from multiple repos. We should add a check to warn if you have divergent branches checked out, similar to the way build --all warns if you are not on the master branch.

[gtback]

This seems like a reasonable request 😉