wireproto: explicitly flush stdio to prevent stalls on Windows

This is the key to fixing the hangs on Windows in D2720[1].  I put flushes in a
bunch of other places that didn't help, but I suspect that's more a lack of test
coverage than anything else.

Chasing down stuff like this is pretty painful.  I'm wondering if we can put a
proxy around sys.stderr (and sys.stdout?) on Windows (only when daemonized?)
that will flush on every write (or at least every write with a '\n').

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-March/113352.html

mercurial/util.py

@@ -715,11 +715,13 b' class fileobjectobserver(object):'
     def _writedata(self, data):
         if not self.logdata:
             self.fh.write('\n')
+            self.fh.flush()
             return
 
         # Simple case writes all data on a single line.
         if b'\n' not in data:
             self.fh.write(': %s\n' % escapedata(data))
+            self.fh.flush()
             return
 
         # Data with newlines is written to multiple lines.
@@ -727,6 +729,7 b' class fileobjectobserver(object):'
         lines = data.splitlines(True)
         for line in lines:
             self.fh.write('%s>     %s\n' % (self.name, escapedata(line)))
+        self.fh.flush()
 
     def read(self, res, size=-1):
         if not self.reads:

mercurial/wireproto.py

@@ -1077,6 +1077,7 b' def unbundle(repo, proto, heads):'
                     util.stderr.write("abort: %s\n" % exc)
                     if exc.hint is not None:
                         util.stderr.write("(%s)\n" % exc.hint)
+                    util.stderr.flush()
                     return pushres(0, output.getvalue() if output else '')
                 except error.PushRaced:
                     return pusherr(pycompat.bytestr(exc),