build: fix build errors caused by chgserver moved to core

Test Plan: `arc diff`

Reviewers: #sourcecontrol, mjpieters

Reviewed By: mjpieters

Subscribers: mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D4244233

Signature: t1:4244233:1480432440:104d3b082b9affddf1d44bb3cfc5f997eb7e9afa

hgext3rd/extorder.py

@@ -26,7 +26,10 @@ will not load them for you
 
 from mercurial import extensions
 from mercurial import error
-from hgext import chgserver
+try:
+    from mercurial import chgserver
+except ImportError:
+    from hgext import chgserver
 chgserver._configsections.append('extorder')
 
 testedwith = 'internal'

hgext3rd/sshaskpass.py

@@ -180,11 +180,14 @@ def _patchchgserver():
     """patch chgserver so we can backup tty fds before they are replaced if
     chg starts the pager.
     """
+    chgserver = None
+    try:
+        from mercurial import chgserver
+    except ImportError:
         try:
             chgserver = extensions.find('chgserver')
         except KeyError:
             pass
-    else:
     server = getattr(chgserver, 'chgcmdserver', None)
     if server and 'attachio' in server.capabilities:
         orig = server.attachio