extensions: prevent a crash on py3 when testing a bad extension minimum

A `None` placeholder is populated for each missing component by
`util.versiontuple()`, which could safely be used with `>` on py2, but not py3.
I guess there's another hole here where if the string is entirely bogus (i.e no
numbers), it will be treated as 0.0, and always load.  But that's always been
the case.

Differential Revision: https://phab.mercurial-scm.org/D11475

mercurial/extensions.py

@@ -224,8 +224,12 b' def load(ui, name, path, loadingtime=Non'
     minver = getattr(mod, 'minimumhgversion', None)
     if minver:
         curver = util.versiontuple(n=2)
+        extmin = util.versiontuple(minver, 2)
 
-        if None in curver or util.versiontuple(minver, 2) > curver:
+        if None in extmin:
+            extmin = (extmin[0] or 0, extmin[1] or 0)
+
+        if None in curver or extmin > curver:
             msg = _(
                 b'(third party extension %s requires version %s or newer '
                 b'of Mercurial (current: %s); disabling)\n'

tests/test-extension.t

@@ -1692,6 +1692,25 b' Can load minimum version identical to cu'
   $ hg --config extensions.minversion=minversion3.py version 2>&1 | egrep '\(third'
   [1]
 
+Don't explode on py3 with a bad version number
+
+  $ cat > minversion4.py << EOF
+  > from mercurial import util
+  > util.version = lambda: b'3.5'
+  > minimumhgversion = b'3'
+  > EOF
+  $ hg --config extensions.minversion=minversion4.py version -v
+  Mercurial Distributed SCM (version 3.5)
+  (see https://mercurial-scm.org for more information)
+  
+  Copyright (C) 2005-* Olivia Mackall and others (glob)
+  This is free software; see the source for copying conditions. There is NO
+  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  
+  Enabled extensions:
+  
+    minversion  external  
+
 Restore HGRCPATH
 
   $ HGRCPATH=$ORGHGRCPATH