chg: suppress OSError in _restoreio() and add some logging (issue6330)

According to issue6330, running chg on heavy loaded systems can lead to
following error:

```
Traceback (most recent call last):
  File "path-to-hg/mercurial/commandserver.py", line 650, in _acceptnewconnection
    self._runworker(conn)
  File "path-to-hg/mercurial/commandserver.py", line 701, in _runworker
    prereposetups=[self._reposetup],
  File "path-to-hg/mercurial/commandserver.py", line 470, in _serverequest
    sv.cleanup()
  File "path-to-hg/mercurial/chgserver.py", line 381, in cleanup
    self._restoreio()
  File "path-to-hg/mercurial/chgserver.py", line 444, in _restoreio
    os.dup2(fd, fp.fileno())
OSError: [Errno 16] Device or resource busy
```

[man dup2] indicates that, on Linux, EBUSY comes from a race condition
between open() and dup2().

However it's not clear why open() race occurred for newfd=stdin/out/err.

We suppress the OSError in _restoreio() since the forked worker process will
finish anyway and add some logging.

Thanks to Mitchell Plamann for a detailed bug description and Yuya Nishihara for
suggesting the fix.

mercurial/chgserver.py

@@ -442,7 +442,20 b' class chgcmdserver(commandserver.server)'
             if newfp is not fp:
                 newfp.close()
             # restore original fd: fp is open again
-            os.dup2(fd, fp.fileno())
+            try:
+                os.dup2(fd, fp.fileno())
+            except OSError as err:
+                # According to issue6330, running chg on heavy loaded systems
+                # can lead to EBUSY. [man dup2] indicates that, on Linux,
+                # EBUSY comes from a race condition between open() and dup2().
+                # However it's not clear why open() race occurred for
+                # newfd=stdin/out/err.
+                self.ui.log(
+                    b'chgserver',
+                    b'got %s while duplicating %s\n',
+                    stringutil.forcebytestr(err),
+                    fn,
+                )
             os.close(fd)
             setattr(self, cn, ch)
             setattr(ui, fn, fp)