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 test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint