node: define experimental identifiers for working directory

The "ff..." node was introduced at 183965a00c76, and we also need an integer
that can be processed in revset. We could use len(repo), but it would be
likely to hide possible bugs. Instead, using INT_MAX, we can notice such bugs
by IndexError, at the cost of handling non-contiguous revisions.

mercurial/node.py

@@ -10,6 +10,11 b' import binascii'
 nullrev = -1
 nullid = "\0" * 20
 
+# pseudo identifiers for working directory
+# (they are experimental, so don't add too many dependencies on them)
+wdirrev = 0x7fffffff
+wdirid = "\xff" * 20
+
 # This ugly style has a noticeable effect in manifest parsing
 hex = binascii.hexlify
 bin = binascii.unhexlify