Remove hard coded single quotes in arc feature

Summary:
Single quotes aren't valid in the windows cmd prompt, so arc feature
didn't work in mercurial when it got to this line.

I have no idea why %C was used before.  Nothing in that string should be
broken by the escaping.

Test Plan:
Ran arc feature --trace on my mac. Verified the command was escaped correctly
and the correct feature results were printed.

I don't have a windows machine to try it on, but the builtin escaping should
now account for windows machines.

Reviewers: epriestley

Reviewed By: epriestley

CC: sid0, aran, Korvin

Differential Revision: https://secure.phabricator.com/D6637