windows: don't return early from building the hgrc search path

This will minimize the changes needed to add other sources.

Differential Revision: https://phab.mercurial-scm.org/D7690

mercurial/scmwindows.py

@@ -38,8 +38,7 b' def systemrcpath():'
     value = util.lookupreg(
         b'SOFTWARE\\Mercurial', None, winreg.HKEY_LOCAL_MACHINE
     )
-    if not isinstance(value, bytes) or not value:
-        return rcpath
+    if value and isinstance(value, bytes):
     value = util.localpath(value)
     for p in value.split(pycompat.ospathsep):
         if p.lower().endswith(b'mercurial.ini'):