simplemerge: move default labels to simplemerge extension
I want to remove knowledge of paths from the `simplemerge` core
module. As part of that, let's move the default labels from the core
module to the extension.
Differential Revision: https://phab.mercurial-scm.org/D11977
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true