dispatch: do not close stdout and stderr, just flush() instead

dispatch: do not close stdout and stderr, just flush() instead

Since 3a4c0905f357 "util: always force line buffered stdout when stdout is
a tty", we have two file objects attached to the same STDOUT_FILENO. If one
is closed, the underlying file descriptor is also closed, and writing to
the other file object would crash the Python interpreter in a hard way, at
least on Windows.

So, it seems safer to not close the standard streams. This also matches
the behavior of the default sys.stdout/stderr.close(), which never close
the FILE* streams in C layer.

https://hg.python.org/cpython/file/v2.7.13/Python/sysmodule.c#l1401

--- a/mercurial/dispatch.py	Mon Jun 05 23:36:35 2017 +0900
+++ b/mercurial/dispatch.py	Sun Apr 23 00:31:29 2017 +0900
@@ -88,13 +88,13 @@
         status = -1
     if util.safehasattr(req.ui, 'fout'):
         try:
-            req.ui.fout.close()
+            req.ui.fout.flush()
         except IOError as err:
             status = -1
     if util.safehasattr(req.ui, 'ferr'):
         if err is not None and err.errno != errno.EPIPE:
             req.ui.ferr.write('abort: %s\n' % err.strerror)
-        req.ui.ferr.close()
+        req.ui.ferr.flush()
     sys.exit(status & 255)
 
 def _getsimilar(symbols, value):