Merge branch 'nd/warn-ambiguous-object-name'

"git cmd <name>", when <name> happens to be a 40-hex string, directly uses the 40-hex string as an object name, even if a ref "refs/<some hierarchy>/<name>" exists. This disambiguation order is unlikely to change, but we should warn about the ambiguity just like we warn when more than one refs/ hierachies share the same name. * nd/warn-ambiguous-object-name: get_sha1: warn about full or short object names that look like refs