`/compare/branches` only works for comparisons involving `master` or a commit

Trying to compare tags or non-master branches results in an empty response, e.g. "No commits to display." or "There are no merged pull requests."

The URLs for these failed comparisons look like this:

.../branches/compare/tag-1.2.3%0Dtag-1.2.4
.../branches/compare/branch-1.2.x%0Dbranch-1.3.x
.../branches/compare/tag-1.2.3%0Dbranch-1.3.x

If either of the fields references a commit or master, the comparisons work as expected.

The raw git commands, e.g. git diff tag-1.2.3 tag-1.2.4, show the expected unified diff output.