Summary:
User reported failures because revlog.opener doesn't have `exists` function.
The reason is that sometimes opener is wrapped inside the function.
Let's use _realopener instead
Test Plan: arc unit
Reviewers: #sourcecontrol, mjpieters
Reviewed By: mjpieters
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D4571659
Signature: t1:4571659:1487267797:a950ab7c88d75573df64ec1964755c054307c7ea
