sha1_name: don't resolve refs when core.warnambiguousrefs is false

sha1_name: don't resolve refs when core.warnambiguousrefs is false

When seeing a full 40-hex object name, get_sha1_basic()
unconditionally checks if the string can also be interpreted as a
refname, but the result will not be used unless warn_ambiguous_refs
is in effect.

Omitting this unnecessary ref resolution provides a substantial
performance improvement, especially when passing many hashes to a
command (like "git rev-list --stdin") and core.warnambiguousrefs is
set to false.  The check incurs 6 stat()s for every hash supplied,
which can be costly over NFS.

Signed-off-by: Brodie Rao <brodie@sf.io>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>