dispatch: ignore further SIGPIPE while handling KeyboardInterrupt

I got the following error by running "hg log" and quitting the pager
immediately. Any output here may trigger another SIGPIPE, so only thing
we can do is to swallow the exception and exit with an error status.

  Traceback (most recent call last):
    File "./hg", line 45, in <module>
      mercurial.dispatch.run()
    File "mercurial/dispatch.py", line 83, in run
      status = (dispatch(req) or 0) & 255
    File "mercurial/dispatch.py", line 167, in dispatch
      req.ui.warn(_("interrupted!\n"))
    File "mercurial/ui.py", line 1224, in warn
      self.write_err(*msg, **opts)
    File "mercurial/ui.py", line 790, in write_err
      self._write_err(*msgs, **opts)
    File "mercurial/ui.py", line 798, in _write_err
      self.ferr.write(a)
    File "mercurial/ui.py", line 129, in _catchterm
      raise error.SignalInterrupt
  mercurial.error.SignalInterrupt

Perhaps this wasn't visible before de5c9d0e02ea because the original stderr
handle was restored very late.

mercurial/dispatch.py

@@ -165,6 +165,10 b' def dispatch(req):'
     except KeyboardInterrupt:
         try:
             req.ui.warn(_("interrupted!\n"))
+        except error.SignalInterrupt:
+            # maybe pager would quit without consuming all the output, and
+            # SIGPIPE was raised. we cannot print anything in this case.
+            pass
         except IOError as inst:
             if inst.errno != errno.EPIPE:
                 raise