This is needed to make Mercurial read the HGEXTENSIONPATH to detect
third-party extensions.  It is called HGEXTENSIONPATH and not
HG_EXTENSION_PATH to keep it consistent with other environment variables for
Mercurial, e.g. HGENCODINGAMBIGUOUS, HGEDITOR ...  Hopefully I or someone else
will get this into Mercurial proper.

diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -13,6 +13,7 @@
 import imp
 import inspect
 import os
+import sys
 
 from .i18n import (
     _,
@@ -108,6 +109,8 @@
 
 def _importh(name):
     """import and return the <name> module"""
+    # Read HGEXTENSIONPATH environment variable when import extensions.
+    sys.path.append(os.getenv("HGEXTENSIONPATH"))
     mod = __import__(pycompat.sysstr(name))
     components = name.split(b'.')
     for comp in components[1:]: