reset: introduce ability to skip updating HEAD

reset: introduce ability to skip updating HEAD

In a subsequent commit we'll introduce a new caller to
`reset_working_tree()` that really only wants to update the index and
working tree, without updating any references. Introduce a new flag that
makes the caller opt in to updating HEAD and adapt all callers to set
that flag.

Note that in a previous iteration we instead introduced a flag that made
callers opt out of updating any references. This was somewhat awkward
though because we already have the `UPDATE_ORIG_HEAD` flag, so the
result was somewhat inconsistent.

Suggested-by: Phillip Wood <redacted>
Signed-off-by: Patrick Steinhardt <redacted>
[jc: fixed-up a typo pointed out by Christian]
Signed-off-by: Junio C Hamano <redacted>