Fix issues with spack [cmd] -h help output

- Shorten Spack command usage for short options. Short options are now
  shown as [-abc] instead of as [-a] [-b] [-c]

- fix bug that mixed long and short options for top-level `spack help`

lib/spack/external/argparse.py

@@ -356,8 +356,6 @@ def _format_usage(self, usage, actions, groups, prefix):
                 pos_usage = format(positionals, groups)
                 opt_parts = _re.findall(part_regexp, opt_usage)
                 pos_parts = _re.findall(part_regexp, pos_usage)
-                assert ' '.join(opt_parts) == opt_usage
-                assert ' '.join(pos_parts) == pos_usage
 
                 # helper for wrapping lines
                 def get_lines(parts, indent, prefix=None):