simplemerge: let filemerge check for binary inputs This is similar to the previous patch, but here we put a specialized copy of `simplemerge._verifytext()` in the the `filemerge` module instead. Differential Revision: https://phab.mercurial-scm.org/D12147

local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed