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

mercurial/simplemerge.py

@@ -499,14 +499,9 b' def simplemerge(ui, localctx, basectx, o'
         extrakwargs[b'name_base'] = name_base
         extrakwargs[b'minimize'] = False
 
-    lines = []
-    for line in m3.merge_lines(
+    lines = m3.merge_lines(
         name_a=name_a, name_b=name_b, **pycompat.strkwargs(extrakwargs)
-    ):
-        if opts.get('print'):
-            ui.fout.write(line)
-        else:
-            lines.append(line)
+    )
 
     # merge flags if necessary
     flags = localctx.flags()
@@ -518,8 +513,10 b' def simplemerge(ui, localctx, basectx, o'
         addedflags = (localflags ^ otherflags) - baseflags
         flags = b''.join(sorted(commonflags | addedflags))
 
-    if not opts.get('print'):
         mergedtext = b''.join(lines)
+    if opts.get('print'):
+        ui.fout.write(mergedtext)
+    else:
         localctx.write(mergedtext, flags)
 
     if m3.conflicts and not mode == b'union':