Prepare saving of commit-ids/tree-ids in reflog

create directory "logs/refs/bup" on bup init
touch logs/refs/bup/latest-{commit,tree}

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>
New function update_ref_force

New function update_ref_force: behaves like update_ref, but without caring
for old content. The refs latest-commit/latest-tree are overwritten each time
(without parent-relationship like branches)

The function is added to lib/bup/git.py (for local BUP_DIRs) as well as
client/server (for remote repositories)

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>
Save most recent commit-id / tree-id

On bup save, save the most recent commit-id / tree-id as refs (into the new/private bup-category)
These are overwritten each time, they are most useful with reflogs.

For git maintaining reflogs for those, the directory logs/refs/bup has to exist already,
so a (re-)init with "bup init" should be run once.

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>