[PATCH V2] localrepo: avoid unnecessary conversion from node to rev

[Jun Wu]

V1 was accepted and in hg-committed [1] already. I don't know the official
recommended workflows but I used to just push follow-up fixes.

In general, it's a good practice to use hg-committed instead of the hg repo
for development, to avoid potential conflicts with other changes.

[1]: https://www.mercurial-scm.org/repo/hg-committed/rev/1791be8a95c5

Excerpts from Stanislau Hlebik's message of 2017-02-07 00:52:13 -0800:
> # HG changeset patch
> # User Stanislau Hlebik <stash at fb.com>
> # Date 1486457478 28800
> #      Tue Feb 07 00:51:18 2017 -0800
> # Node ID 8614d5d15d6fe47c3bcfdf79823f68f18c7741ae
> # Parent  1f51b4658f21bbb797e922d155c1046eddccf91d
> localrepo: avoid unnecessary conversion from node to rev
> 
> changelog.heads() first calls headrevs then converts them to nodes.
> localrepo.heads() then sorts them using self.changelog.rev function and makes
> useless conversion back to revs. Instead let's call changelog.headrevs() from
> localrepo.heads(), sort the output and then convert to nodes. Because headrevs
> does not support start parameter this optimization only works if start is None.
> 
> diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
> --- a/mercurial/localrepo.py
> +++ b/mercurial/localrepo.py
> @@ -1852,6 +1852,10 @@
>                                    listsubrepos)
>  
>      def heads(self, start=None):
> +        if start is None:
> +            headrevs = reversed(self.changelog.headrevs())
> +            return [self.changelog.node(rev) for rev in headrevs]
> +
>          heads = self.changelog.heads(start)
>          # sort the output in rev descending order
>          return sorted(heads, key=self.changelog.rev, reverse=True)