Allow external use of Command.usage() and use CmdOptionParser.set_usage()

This fixes
  $ python be diff -2
  Usage: be [options]
  
  be: error: no such option: -2
and we now get the correct output
  $ python be diff -2
  Usage: be diff [options] [REVISION]
  
  be: error: no such option: -2

1 files changed, 3 insertions, 1 deletions

diff --git a/libbe/ui/command_line.py b/libbe/ui/command_line.py
index 856f810..b5a3991 100755
--- a/libbe/ui/command_line.py
+++ b/libbe/ui/command_line.py
@@ -48,6 +48,8 @@ class CmdOptionParser(optparse.OptionParser):
         self._option_by_name = {}
         for option in self.command.options:
             self._add_option(option)
+        self.set_usage(command.usage())
+
 
     def _add_option(self, option):
         option.validate()
@@ -215,7 +217,7 @@ class BE (libbe.command.Command):
                     name='args', optional=True, repeatable=True)
                 ])
 
-    def _usage(self):
+    def usage(self):
         return 'usage: be [options] [COMMAND [command-options] [COMMAND-ARGS ...]]'
 
     def _long_help(self):