branches: populate all template keywords in formatter

This is a usage example of fm.context().

mercurial/commands.py

@@ -1258,6 +1258,7 b' def branches(ui, repo, active=False, clo'
         fmt = ' ' * padsize + ' %d:%s'
         fm.condwrite(not ui.quiet, 'rev node', fmt, rev, hexfunc(ctx.node()),
                      label='log.changeset changeset.%s' % ctx.phasestr())
+        fm.context(ctx=ctx)
         fm.data(active=isactive, closed=not isopen, current=current)
         if not ui.quiet:
             fm.plain(notice)

tests/test-branches.t

@@ -519,6 +519,12 b' template output:'
   $ hg branches --closed -T '{if(closed, "{branch}\n")}'
   c
 
+  $ hg branches -T '{word(0, branch)}: {desc|firstline}\n'
+  b: reopen branch with a change
+  a: Adding d branch
+  a: Adding b branch head 2
+  default: Adding root node
+
 Tests of revision branch name caching
 
 We rev branch cache is updated automatically. In these tests we use a trick to