reset: allow the caller to specify the current HEAD object

reset: allow the caller to specify the current HEAD object

When calling `reset_working_tree()` 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 <redacted>
Signed-off-by: Junio C Hamano <redacted>