reset: allow the caller to specify the current HEAD object
When calling `reset_head()` we automatically derive the commit that the
callers wants to move from by reading the HEAD commit. Some callers may
already have resolved it, or they may want to move from a different
commit that doesn't match HEAD.
Introduce a new `oid_from` option that lets the caller specify the
commit.
Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>