t0302: test credential-store support for XDG_CONFIG_HOME

t0302: test credential-store support for XDG_CONFIG_HOME

t0302 now tests git-credential-store's support for the XDG user-specific
configuration file $XDG_CONFIG_HOME/git/credentials. Specifically:

* Ensure that the XDG file is strictly opt-in. It should not be created
  by git at all times if it does not exist.

* Conversely, if the XDG file exists, ~/.git-credentials should
  not be created at all times.

* If both the XDG file and ~/.git-credentials exists, then both files
  should be used for credential lookups. However, credentials should
  only be written to ~/.git-credentials.

* Credentials must be erased from both files.

* $XDG_CONFIG_HOME can be a custom directory set by the user as per the
  XDG base directory specification. Test that git-credential-store
  respects that, but defaults to "~/.config/git/credentials" if it does
  not exist or is empty.

Helped-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>