simplemerge: write output only once it's complete
`simplemerge()` can write either to `ui.fout` or to the file context
(for in-memory merge). This patch simplifies the code a bit by making
it build the output the same way regardless of where it's written, and
then writes the whole output at once. I don't think it will be a
problem that we don't output anything until the whole file is merged
even if the file is large.
Differential Revision: https://phab.mercurial-scm.org/D9550