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>