heads: add support for specifying branches by revset

Before this commit, e.g. "hg heads .^" would fail with:

  abort: unknown revision '.^'!

Like the previous patch, I don't care about the command itself (I
don't think I had ever used it before), I'm just cleaning up uses of
repo[<string>].

Differential Revision: https://phab.mercurial-scm.org/D3084

mercurial/commands.py

@@ -2648,7 +2648,8 b' def heads(ui, repo, *branchrevs, **opts)'
         heads = [repo[h] for h in heads]
 
     if branchrevs:
-        branches = set(repo[br].branch() for br in branchrevs)
+        branches = set(repo[r].branch()
+                       for r in scmutil.revrange(repo, branchrevs))
         heads = [h for h in heads if h.branch() in branches]
 
     if opts.get('active') and branchrevs: