demandimport: disable if chg is being used

In chg's case, making modules lazily loaded could actually slow down things
since chg pre-imports them. Therefore disable demandimport if chg is being
used.

This is not done by setting `HGDEMANDIMPORT` chg client-side because that
has side-effects on child processes (hooks, etc).

Differential Revision: https://phab.mercurial-scm.org/D351

hgdemandimport/__init__.py

@@ -73,5 +73,7 b' disable = demandimport.disable'
 deactivated = demandimport.deactivated
 
 def enable():
-    if os.environ.get('HGDEMANDIMPORT') != 'disable':
+    # chg pre-imports modules so do not enable demandimport for it
+    if ('CHGINTERNALMARK' not in os.environ
+        and os.environ.get('HGDEMANDIMPORT') != 'disable'):
         demandimport.enable()

mercurial/chgserver.py

@@ -565,8 +565,11 b' class chgunixservicehandler(object):'
                             self._hashstate, self._baseaddress)
 
 def chgunixservice(ui, repo, opts):
-    # CHGINTERNALMARK is temporarily set by chg client to detect if chg will
-    # start another chg. drop it to avoid possible side effects.
+    # CHGINTERNALMARK is set by chg client. It is an indication of things are
+    # started by chg so other code can do things accordingly, like disabling
+    # demandimport or detecting chg client started by chg client. When executed
+    # here, CHGINTERNALMARK is no longer useful and hence dropped to make
+    # environ cleaner.
     if 'CHGINTERNALMARK' in encoding.environ:
         del encoding.environ['CHGINTERNALMARK']