R1:5660684d7f44

Never use "{branches}" in Mercurial

Summary:
Fixes T5304. Mercurial features a "{branches}" template keyword, documented as:

```
branches List of strings. The name of the branch on which the
changeset was committed. Will be empty if the branch name
was default.
```

At some time long in the past, I misinterpreted this to mean "list of branches where the branch head is a descendant of the commit". It is more like "list of zero or one elements, possibly…