Never use "{branches}" in Mercurial
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Description

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 It is more like "list of zero or one elements, possibly containing the name of the branch the commit was originally made to, if that branch was not 'default'".

In fact, it seems like this is because a very long time in the past, Mercurial worked roughly like I expected:

Ages ago (2005), we had a very different and ultimately unworkable
approach to named branches that worked vaguely like .hgtags and allowed
multiple branch names per revision.

http://marc.info/?l=mercurial-devel&m=129883069414855

This appears to be deprecated in modern Mercurial (it's not in the modern web documentation) although I can't find a commit about it so maybe that's just a documentation issue.

In any case, {branches} seems to never be useful: {branch} provides the same information without the awkward "default-if-empty" case.

Switch from {branches} to either {branch} (where that's good enough, notably in the hook engine) or (descendants(%s) and head()), which is equivalent to --contains in Git.

This fixes pushing to branches with spaces in their names, and makes the "Branches" / "Contains" queries moderately more consistent.

Test Plan: