simplemerge: convert `merge_lines()` away from generator

Authored by martinvonz.


simplemerge: convert merge_lines() away from generator

We always consume all the lines and put them in a list anyway. By
making the function not a generator, we can later make it return an
additional value (to indicate if there were conflicts).

Differential Revision: https://phab.mercurial-scm.org/D11973