demandimport: make it possible to disable by setting HGDEMANDIMPORT=disable

Convenient for debugging weird problems that are caused by demandimport or
obfuscated by it.

This is an undocumented developer feature.

mercurial/demandimport.py

@@ -24,7 +24,7 b' These imports will not be delayed:'
   b = __import__(a)
 '''
 
-import __builtin__
+import __builtin__, os
 _origimport = __import__
 
 nothing = object()
@@ -167,7 +167,8 b' def isenabled():'
 
 def enable():
     "enable global demand-loading of modules"
-    __builtin__.__import__ = _demandimport
+    if os.environ.get('HGDEMANDIMPORT') != 'disable':
+        __builtin__.__import__ = _demandimport
 
 def disable():
     "disable global demand-loading of modules"

tests/test-demandimport.py

@@ -37,3 +37,9 b' print "fred =", f(fred)'
 print "re =", f(re)
 print "re.stderr =", f(re.stderr)
 print "re =", f(re)
+
+demandimport.disable()
+os.environ['HGDEMANDIMPORT'] = 'disable'
+demandimport.enable()
+from mercurial import node
+print "node =", f(node)

tests/test-demandimport.py.out

@@ -13,3 +13,4 b" fred = <proxied module 're'>"
 re = <unloaded module 'sys'>
 re.stderr = <open file '<whatever>', mode 'w' at 0x?>
 re = <proxied module 'sys'>
+node = <module 'mercurial.node' from '?'>