ssh: fix flakey ssh errors on BSD systems stable

ssh: fix flakey ssh errors on BSD systems

This is a trivial backport of c037fd655b47 performed by
[e-mail], but the change is still really Durham's not mine, so
I [augie] am leaving him as the author.

--- a/mercurial/sshpeer.py	Thu Sep 14 11:16:57 2017 -0700
+++ b/mercurial/sshpeer.py	Mon Sep 11 15:59:18 2017 -0700
@@ -190,23 +190,33 @@
         self.pipei = doublepipe(self.ui, self.pipei, self.pipee)
         self.pipeo = doublepipe(self.ui, self.pipeo, self.pipee)
 
-        # skip any noise generated by remote shell
-        self._callstream("hello")
-        r = self._callstream("between", pairs=("%s-%s" % ("0"*40, "0"*40)))
+        def badresponse():
+            self._abort(error.RepoError(_('no suitable response from '
+                                          'remote hg')))
+
+        try:
+            # skip any noise generated by remote shell
+            self._callstream("hello")
+            r = self._callstream("between", pairs=("%s-%s" % ("0"*40, "0"*40)))
+        except IOError:
+            badresponse()
+
         lines = ["", "dummy"]
         max_noise = 500
         while lines[-1] and max_noise:
-            l = r.readline()
-            self.readerr()
-            if lines[-1] == "1\n" and l == "\n":
-                break
-            if l:
-                self.ui.debug("remote: ", l)
-            lines.append(l)
-            max_noise -= 1
+            try:
+                l = r.readline()
+                self.readerr()
+                if lines[-1] == "1\n" and l == "\n":
+                    break
+                if l:
+                    self.ui.debug("remote: ", l)
+                lines.append(l)
+                max_noise -= 1
+            except IOError:
+                badresponse()
         else:
-            self._abort(error.RepoError(_('no suitable response from '
-                                          'remote hg')))
+            badresponse()
 
         self._caps = set()
         for l in reversed(lines):