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

@@ -14,8 +14,12 b' from mercurial import ('
 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: