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
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors