Perforce repositories work differently from all other types of

repositories. Instead of all communication happening over the Path, it

happens over the Mirror Path. This is done for very ancient historical

reasons, centered around a very early need for tools like Review Board

using a read-only Perforce mirror that was separate from the main

read-write repository that developers had been using. This inversed

logic is sadly probably here to stay long-term.
The problem is, the repository form didn't know about Perforce's needs

here, and was checking only the Path field, not Mirror Path, when

validating a repository. This was generally fine for most usage, but if

using an SSL-backed repository in Mirror Path that had to be explicitly

trusted, Review Board would never have an opportunity to validate this

and present it to the user. The only way it'd work is to have the SSL

repository in Path, and a non-SSL in Mirror Path, at which point the

communication wouldn't be encrypted.
This change fixes this up by adding a new SCMTool.prefers_mirror_path

flag, which instructs the form to check using the Mirror Path if set.

Only Perforce has this enabled, and the documentation discourages others

from ever setting it.
In time, we can clean some of this up at a presentational level by

creating a custom subform and making it clear which is used for which.

This change does take a step toward this by adding new help text for

Mirror Path that makes it clear it takes precedence for communication.

Unit tests pass.
Manually tested adding a local repository and verifying the paths it

was using.