options.py: make --usage just print the usage message.

This is a relatively common option in other programs, so let's make it work
in case someone tries to use it.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>

lib/bup/options.py

@@ -115,7 +115,7 @@ def __init__(self, optspec, optfunc=getopt.gnu_getopt,
         self.optfunc = optfunc
         self._aliases = {}
         self._shortopts = 'h?'
-        self._longopts = ['help']
+        self._longopts = ['help', 'usage']
         self._hasparms = {}
         self._defaults = {}
         self._usagestr = self._gen_usage()
@@ -214,7 +214,7 @@ def parse(self, args):
 
         for (k,v) in flags:
             k = k.lstrip('-')
-            if k in ('h', '?', 'help'):
+            if k in ('h', '?', 'help', 'usage'):
                 self.usage()
             if k.startswith('no-'):
                 k = self._aliases[k[3:]]