Add flag to ignore changes of current user for list subcommand

This commit adds new flag to list subcommand, --no-mine AKA -M. When
it's specified, gerda will not include changes submitted or already
approved by current user to the list.
Improve python2.6 compatibility

- in python 2.6, str.encode() does not accept kwargs, so we have
  to use positional arguments only;
- use exception.args[0] instead of message, because using message
  leads to deprecation warning.
Better compatibility with older git

In older git releases 'git symbolic-ref' did not have '--short'
option, so we implement shortening ourselves.