ref updates: Introduce update_ref_cmd

ref updates: Introduce update_ref_cmd

Currently the comment about `message may be modified' is rather
misleading, since there is no code to do that - but it explains the
semantics of the new function.

We change the appropriate subset of the git update-ref call sites.
These are precisely the ones where we previously specified a message
with -m, except for the quilt cache (where we use the reflog message
as the cache lookup key).

The calls are updates of internal refs etc.

We end up using qw(git) rather than @git.
Otherwise no functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>