get_diff_hg: Disable color extension

This handles the case where a user forces colors from the color extension via
hgrc:

    [extensions]
    color =

    [defaults]
    diff = --colors=always

Sy disables these for its 'hg diff' call, otherwise the output would be
riddled with color escape sequences and couldn't be parsed properly which
would result in no signs shown.

References #146.