Fix command output

The first line lacked the subcommand like:

$ gbp pull --help
Usage: gbp [options] - safely update a repository from remote

instead of

$ gbp pull --help
Usage: gbp pull [options] - safely update a repository from remote
           ^^^^

1 files changed, 7 insertions, 1 deletions

diff --git a/gbp/config.py b/gbp/config.py
index fa6e679..9ac1ee8 100644
--- a/gbp/config.py
+++ b/gbp/config.py
@@ -20,6 +20,7 @@ from optparse import OptionParser, OptionGroup, Option, OptionValueError
 from ConfigParser import SafeConfigParser, NoSectionError
 from copy import copy
 import os.path
+import sys
 try:
     from gbp.version import gbp_version
 except ImportError:
@@ -356,7 +357,7 @@ class GbpOptionParser(OptionParser):
         else:
             self.config['filter'] = []
 
-    def __init__(self, command, prefix='', usage=None, sections=[]):
+    def __init__(self, command=None, prefix='', usage=None, sections=[]):
         """
         @param command: the command to build the config parser for
         @type command: C{str}
@@ -368,6 +369,10 @@ class GbpOptionParser(OptionParser):
             to parse
         @type sections: C{list} of C{str}
         """
+        if not command:
+            command = os.path.basename(sys.argv[0])
+            if command == 'gbp':
+                command += " " + os.path.basename(sys.argv[1])
         self.command = command
         self.sections = sections
         self.prefix = prefix
@@ -375,6 +380,7 @@ class GbpOptionParser(OptionParser):
         self.config_files = self.get_config_files()
         self._parse_config_files()
         OptionParser.__init__(self, option_class=GbpOption,
+                              prog=command,
                               usage=usage, version='%s %s' % (self.command,
                                                               gbp_version))