revlog: use a less probable revlog version for backwards compatibility

This allows us to define a usable v2.

Differential Revision: https://phab.mercurial-scm.org/D9902

mercurial/localrepo.py

@@ -3639,11 +3639,11 b' def createrepository(ui, path, createopt'
         # effectively locks out old clients and prevents them from
         # mucking with a repo in an unknown format.
         #
-        # The revlog header has version 2, which won't be recognized by
+        # The revlog header has version 65535, which won't be recognized by
         # such old clients.
         hgvfs.append(
             b'00changelog.i',
-            b'\0\0\0\2 dummy changelog to prevent using the old repo '
+            b'\0\0\xFF\xFF dummy changelog to prevent using the old repo '
             b'layout',
         )
 

tests/test-requires.t

@@ -5,7 +5,7 b''
   $ hg commit -m test
   $ rm .hg/requires
   $ hg tip
-  abort: unknown version (2) in revlog 00changelog.i
+  abort: unknown version (65535) in revlog 00changelog.i
   [50]
   $ echo indoor-pool > .hg/requires
   $ hg tip