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

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