templater: makes branches work correctly with stringify (issue4108)

--HG--
branch : stable
test-command-template.t: fix failure due to trailing whitespace
help: drop help for branches template keyword

The {branches} keyword dates to pre-1.0 Mercurial's tag-like branch
scheme which allowed changesets to be on multiple branches. This is
the last visible vestige of that scheme, users should instead be using
{branch}, possibly with if().

--HG--
extra : rebase_source : 090257a171293d90410368dc8a4c340b757f56d0
merge with stable