hghave: disable demandimport when chg is running

Chg disables demandimport on purpose for performance wins and
therefore, it probably makes sense to indicate that demandimport is disabled
when chg is running.

Test Plan:
Ran all the tests.

Differential Revision: https://phab.mercurial-scm.org/D1161

tests/hghave.py

@@ -601,7 +601,8 b' def has_debdeps():'
 
 @check("demandimport", "demandimport enabled")
 def has_demandimport():
-    return os.environ.get('HGDEMANDIMPORT') != 'disable'
+    # chg disables demandimport intentionally for performance wins.
+    return ((not has_chg()) and os.environ.get('HGDEMANDIMPORT') != 'disable')
 
 @check("py3k", "running with Python 3.x")
 def has_py3k():