Create pull request incorrectly says "no changes" when the real problem is conflicts (BB-9181)

A colleage had a single commit that she wanted to make a PR from, but the "Create Pull Request" button was disabled and the page said "No changes". When switching to the "Compare" tab, it finds the commit, but a little warning icon indicates that there are conflicts between her commit and the tip.

We then merged the commit locally, resolving the conflict and pushed the merge commit, at which point the "Pull Request" tab successfully detects the (now 2) commits and allows us to open a PR.

Please either provide an option to make an initial PR that can't be auto-merged (we can always update the PR later with a manual merge commit), or at the very least don't tell the user there are no changes when in fact there is a commit, but it can't merge cleanly.