simplemerge: let extension check for binary inputs (unless `--text`)
The `simplemerge` module does too much UI-related things. This patch
puts a specialized copy of `simplemerge._verifytext()` in the
simplemerge extension.
Differential Revision: https://phab.mercurial-scm.org/D12146
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true