Better heuristic for checking for hgsubversion

Summary:
The existing heuristic checks for the existence of .hg/svn, but it
turns out that this directory can exist in a non-svn hg repo if the user or a
script ever runs a 'hg svn' command.

Now we check for a file inside .hg/svn.  The hgsubversion maintainer said this
particular file will always be present in hgsubversion repos.

Test Plan:
arc land --trace
Verified it used 'hg push' and not 'hg push -r ...'. This indicates it
considered the repo to be an hgsubversion repo.

Reviewers: epriestley

Reviewed By: epriestley

CC: dschleimer, sid0, Korvin, aran

Differential Revision: https://secure.phabricator.com/D6807