[PATCH 1 of 4] revset: fix crash on empty sort key

[Jun Wu]

Looks good to me.

Excerpts from Yuya Nishihara's message of 2016-06-15 22:42:52 +0900:
> # HG changeset patch
> # User Yuya Nishihara <yuya at tcha.org>
> # Date 1465990644 -32400
> #      Wed Jun 15 20:37:24 2016 +0900
> # Node ID 05e51f240485ea6a8490342deda2ce1f08c33bbf
> # Parent  8bf84295e59ba0c98e64fd7ccf3f809e4da5b237
> revset: fix crash on empty sort key
> 
> Make it noop as before 2188f170f5b6. We could change it to an error, but
> allowing empty key makes some sense for scripting that builds a key string
> programmatically.
> 
> diff --git a/mercurial/revset.py b/mercurial/revset.py
> --- a/mercurial/revset.py
> +++ b/mercurial/revset.py
> @@ -1890,6 +1890,8 @@ def sort(repo, subset, x):
>                  'topo.firstbranch can only be used when using the topo sort '
>                  'key'))
>  
> +    if not keys:
> +        return revs
>      if keys == ["rev"]:
>          revs.sort()
>          return revs
> diff --git a/tests/test-revset.t b/tests/test-revset.t
> --- a/tests/test-revset.t
> +++ b/tests/test-revset.t
> @@ -962,6 +962,13 @@ test sorting two sorted collections in d
>    6
>    2
>  
> +test empty sort key which is noop
> +
> +  $ log 'sort(0 + 2 + 1, "")'
> +  0
> +  2
> +  1
> +
>  test invalid sort keys
>  
>    $ log 'sort(all(), -invalid)'