dumprevlog: handle being passed a mode parameter

This makes it closer to an actual file opener. Otherwise, it gets in the way
of a coming refactoring.

contrib/dumprevlog

@@ -1,31 +1,35 b''
 #!/usr/bin/env python
 # Dump revlogs as raw data stream
 # $ find .hg/store/ -name "*.i" | xargs dumprevlog > repo.dump
 
 from __future__ import absolute_import, print_function
 
 import sys
 from mercurial import (
     node,
     revlog,
     util,
 )
 
 for fp in (sys.stdin, sys.stdout, sys.stderr):
     util.setbinary(fp)
 
+def binopen(path, mode='rb'):
+    if 'b' not in mode:
+        mode = mode + 'b'
+    return open(path, mode)
+
 for f in sys.argv[1:]:
-    binopen = lambda fn: open(fn, 'rb')
     r = revlog.revlog(binopen, f)
     print("file:", f)
     for i in r:
         n = r.node(i)
         p = r.parents(n)
         d = r.revision(n)
         print("node:", node.hex(n))
         print("linkrev:", r.linkrev(i))
         print("parents:", node.hex(p[0]), node.hex(p[1]))
         print("length:", len(d))
         print("-start-")
         print(d)
         print("-end-")