_get_this_repo fails if user is not in top level of git repo

[Robert C Jennings]

When git lp-propose is run from a sub directory below the root of the repo we get a traceback. As a user I can ask git to find the root of the tree using 'git rev-parse --show-toplevel'. And for your initialization of git.Repo(... you can pass 'search_parent_directories=True' to resolve this.

$ git clone https://git.launchpad.net/gitlptools
$ cd gitlptools/src/gitlptools
$ git lp-propose ....
Traceback (most recent call last):
  File "/snap/gitlptools/37/bin/git-lp-propose", line 11, in <module>
    load_entry_point('gitlptools==1.0', 'console_scripts', 'git-lp-propose')()
  File "/snap/gitlptools/37/lib/python3.6/site-packages/gitlptools/__init__.py", line 210, in git_lp_propose
    options = parse_args_with_logging(parser, argv)
  File "/snap/gitlptools/37/lib/python3.6/site-packages/gitlptools/__init__.py", line 411, in parse_args_with_logging
    options = parser.parse_args(argv)
  File "/snap/gitlptools/37/usr/lib/python3.6/argparse.py", line 1743, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/snap/gitlptools/37/usr/lib/python3.6/argparse.py", line 1775, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/snap/gitlptools/37/usr/lib/python3.6/argparse.py", line 1984, in _parse_known_args
    stop_index = consume_positionals(start_index)
  File "/snap/gitlptools/37/usr/lib/python3.6/argparse.py", line 1940, in consume_positionals
    take_action(action, args)
  File "/snap/gitlptools/37/usr/lib/python3.6/argparse.py", line 1849, in take_action
    action(self, namespace, argument_values, option_string)
  File "/snap/gitlptools/37/lib/python3.6/site-packages/gitlptools/__init__.py", line 50, in __call__
    merge_target = get_merge_target(_get_this_repo(), values)
  File "/snap/gitlptools/37/lib/python3.6/site-packages/gitlptools/__init__.py", line 393, in _get_this_repo
    return git.Repo(repo_path)
  File "/snap/gitlptools/37/lib/python3.6/site-packages/git/repo/base.py", line 181, in __init__
    raise InvalidGitRepositoryError(epath)
git.exc.InvalidGitRepositoryError: /home/fake/gitlptools/src/gitlptools