attr: fix BUG() when parsing attrs outside of repo

attr: fix BUG() when parsing attrs outside of repo

If either the `--attr-source` option or the `GIT_ATTR_SOURCE` envvar are
set, then `compute_default_attr_source()` will try to look up the value
as a treeish. It is possible to hit that function while outside of a Git
repository though, for example when using `git grep --no-index`. In that
case, Git will hit a bug because we try to look up the main ref store
outside of a repository.

Handle the case gracefully and detect when we try to look up an attr
source without a repository.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>