The way that the initial commit was represented when fetching lists of

commits for Mercurial repositories wasn't correct, reporting an ID of 40

consecutive zeros, instead of an empty parent ID. The result of this is

that when the "New Review Request" UI hit the end of the commit list, it

would end up in an infinite loop requesting new commits starting at

00000....
This change fixes all affected backends to convert the 000 commit to a

proper empty string.

Loaded the /r/new/ UI for a tiny Mercurial repository and saw that it

  no longer made infinite requests for commits.
Ran unit tests.