(experimental) Work around a Windows escaping issue and security conecern in "hg cat --output ..."

Summary:
See PHI904. Ref T13210. Ref T13209. Currently, we have an hg cat construction which attempts to pass a literal %p to Mercurial. This fails because you can't pass % through %s outside of wilds.

It also uses %C to pass a list of file paths. This is broadly unsafe and can cause command execution if you modify a file named, e.g., ; rm -rf xyz or similar. I think it would be difficult to turn this into an attack but it's fairly bad. This dates from D5144 in 2013.

Test Plan: With this patch, created D19757 which has valid binary data (see F5962134).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210, T13209

Differential Revision: https://secure.phabricator.com/D19758