simplemerge: move printing of merge result to extension The `mercurial.simplemerge` module still has some command-lines processing that doesn't belong in such a low-level module. This patch moves the handling of `hg simplemerge --print` to the extension by having `mercurial.simplemerge.simplemerge()` return the merged text. Differential Revision: https://phab.mercurial-scm.org/D12148

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls -A
  .hg
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls -A
  .hg
  a