simplemerge: make merge_lines() a free function
IMO, the rendering of a conflict as text with merge markers should be
separate from the code for resolving conflicts. The latter is what
`Merge3Text` is mostly about already.
Differential Revision: https://phab.mercurial-scm.org/D11974
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors