ssh: add capability detection at startup

Because older servers don't return any output for unknown commands,
it's tricky to add new commands. The approach is this: we add a
"hello" command that reports any interesting capabilities (and other
things that might be of interest in the future). To detect whether
this new command is supported, we issue both it and our startup
detection command ("between") at the beginning of a connection.

mercurial/sshrepo.py

@@ -37,6 +37,7 b' class sshrepository(remoterepository):'
         self.pipeo, self.pipei, self.pipee = os.popen3(cmd, 'b')
 
         # skip any noise generated by remote shell
+        self.do_cmd("hello")
         r = self.do_cmd("between", pairs=("%s-%s" % ("0"*40, "0"*40)))
         lines = ["", "dummy"]
         max_noise = 500
@@ -54,6 +55,13 b' class sshrepository(remoterepository):'
                 ui.debug(_("remote: "), l1)
             raise hg.RepoError(_("no response from remote hg"))
 
+        self.capabilities = ()
+        lines.reverse()
+        for l in lines:
+            if l.startswith("capabilities:"):
+                self.capabilities = l[:-1].split(":")[1].split()
+                break
+
     def readerr(self):
         while 1:
             size = util.fstat(self.pipee).st_size