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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true