Improve 'rbt patch' usage for Hg by overriding apply_patch()

By default 'rbt patch' uses GNU patch with a prefix value of 0. With Mercurial,
patches often fail to apply because they're fetched from Review Board with the
git-style prefixes 'a/' and 'b/'. Also, on Windows the binaries available are
very outdated and the only way to get a new version is to install Cygwin.

Avoid these problems by overriding apply_patch() and running 'hg patch' instead.

Reviewed at https://reviews.reviewboard.org/r/6012/