submodule: try harder to fetch needed sha1 by direct fetching sha1

When reviewing a change that also updates a submodule in Gerrit, a common review practice is to download and cherry-pick the patch locally to test it. However when testing it locally, the 'git submodule update' may fail fetching the correct submodule sha1 as the corresponding commit in the submodule is not yet part of the project history, but also just a proposed change. If $sha1 was not part of the default fetch, we try to fetch the $sha1 directly. Some servers however do not support direct fetch by sha1, which leads git-fetch to fail quickly. We can fail ourselves here as the still missing sha1 would lead to a failure later in the checkout stage anyway, so failing here is as good as we can get. Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>