get_sha1: warn about full or short object names that look like refs

get_sha1: warn about full or short object names that look like refs

When we get 40 hex digits, we immediately assume it's an SHA-1. This
is the right thing to do because we have no way else to specify an
object. If there is a ref with the same object name, it will be
ignored. Warn the user about this case because the ref with full
object name is likely a mistake, for example

    git checkout -b $empty_var $(git rev-parse something)

advice.object_name_warning is not documented because frankly people
should not be aware about it until they encounter this situation.

While at there, warn about ambiguation with abbreviated SHA-1 too.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>