remote: avoid reading $GIT_DIR config in non-repo

remote: avoid reading $GIT_DIR config in non-repo

The "git ls-remote" command can be run outside of a
repository, but needs to look up configured remotes. The
config code is smart enough to handle this case itself, but
we also check the historical "branches" and "remotes" paths
in $GIT_DIR. The git_path() function causes us to blindly
look at ".git/remotes", even if we know we aren't in a git
repository.

For now, this is just an unlikely bug (you probably don't
have such a file if you're not in a repository), but it will
become more obvious once we merge b1ef400ee (setup_git_env:
avoid blind fall-back to ".git", 2016-10-20):

  [now]
  $ git ls-remote
  fatal: No remote configured to list refs from.

  [with b1ef400ee]
  $ git ls-remote
  fatal: BUG: setup_git_env called without repository

We can fix this by skipping these sources entirely when
we're outside of a repository.

The test is a little more complex than the demonstration
above. Rather than detect the correct behavior by parsing
the error message, we can actually set up a case where the
remote name we give is a valid repository, but b1ef400ee
would cause us to die in the configuration step.

This test doesn't fail now, but it future-proofs us for the
b1ef400ee change.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>