windows: use abspath in extensions

We replace `os.path.abspath` with `util.abspath`. This should solve more "drive
capitalization" issue on Windows.

Differential Revision: https://phab.mercurial-scm.org/D11062

mercurial/extensions.py

@@ -713,7 +713,7 b' def _disabledpaths():'
     # it might not be on a filesystem even if it does.
     if util.safehasattr(hgext, '__file__'):
         extpath = os.path.dirname(
-            os.path.abspath(pycompat.fsencode(hgext.__file__))
+            util.abspath(pycompat.fsencode(hgext.__file__))
         )
         try:
             files = os.listdir(extpath)