Update arc-hg to support mercurial 6.1

Summary:
Updated the mercurial script to pull the parseurl function from the new module if pulling from the old module fails.

Also updated pulling of remoteopts to follow the same pattern of fallback.

Fixes T13672

Test Plan:
Using mercurial 6.1 I ran arc patch Dnnnnn --trace and verified that it succeeded and in the trace output it used the arc-ls-markers extension.

Using mercurial 4.7 I ran arc patch Dnnnnn --trace and verified that it succeeded and in the trace output it used the arc-ls-markers extension.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T13672

Differential Revision: https://secure.phabricator.com/D21747