push, fetch: error out for submodule entries not pointing to commits

push, fetch: error out for submodule entries not pointing to commits

The check_has_commit helper uses resolves a submodule entry to a
commit, when validating its existence. As a side effect this means
tolerates a submodule entry pointing to a tag, which is not a valid
submodule entry that git commands would know how to cope with.

Tighten the check to require an actual commit, not a tag pointing to a
commit.

Also improve the error handling when a submodule entry points to
non-commit (e.g., a blob) to error out instead of warning and
pretending the pointed to object doesn't exist.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>