completion tests: add tests for the __git_refs() helper function

completion tests: add tests for the __git_refs() helper function

Check how __git_refs() lists refs in different scenarios, i.e.

  - short and full refs,
  - from a local or from a remote repository,
  - remote specified via path, name or URL,
  - with or without a repository specified on the command line,
  - non-existing remote,
  - unique remote branches for 'git checkout's tracking DWIMery,
  - not in a git repository, and
  - interesting combinations of the above.

Seven of these tests expect failure, mostly demonstrating bugs related
to listing refs from a remote repository:

  - ignoring the repository specified on the command line (2 tests),
  - listing refs from the wrong place when the name of a configured
    remote happens to match a directory,
  - listing only 'HEAD' but no short refs from a remote given as URL,
  - listing 'HEAD' even from non-existing remotes (2 tests), and
  - listing 'HEAD' when not in a repository.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>