scmutil: add function to help handling workingctx in arithmetic operation

It's unfortunate that workingctx revision is None, which doesn't work well in
arithmetic operation or comparison. This function is trivial but will be used
in several places.

mercurial/scmutil.py

@@ -582,6 +582,13 b' def rcpath():'
             _rcpath = osrcpath()
     return _rcpath
 
+def intrev(repo, rev):
+    """Return integer for a given revision that can be used in comparison or
+    arithmetic operation"""
+    if rev is None:
+        return len(repo)
+    return rev
+
 def revsingle(repo, revspec, default='.'):
     if not revspec and revspec != 0:
         return repo[default]