Add --version flag to redo.

Something to this effect would be useful, though I'm sure my
install.do changes are not how you'd do it.

(Slightly modified by apenwarr)

redo.py

@@ -14,12 +14,17 @@ k,keep-going  keep going as long as possible even if some targets fail
 shuffle    randomize the build order to find dependency bugs
 debug-locks  print messages about file locking (useful for debugging)
 debug-pids   print process ids as part of log messages (useful for debugging)
+version    print the current version and exit
 """
 o = options.Options('redo', optspec)
 (opt, flags, extra) = o.parse(sys.argv[1:])
 
 targets = extra or ['all']
 
+if opt.version:
+    import version
+    print version.TAG
+    sys.exit(0)
 if opt.debug:
     os.environ['REDO_DEBUG'] = str(opt.debug or 0)
 if opt.verbose: