add a tag command

add a tag command

Currently implemented: list all tags, add a tag on a specific commit or
head, delete a known tag.

Also, make vfs expose a new directory called '/.tag' which contains a
link for each tag to its associated commit directory situated in
'/.commit'. Finally, make tags appear as symlinks on branch directories
on which the tagged commit is visible.

Signed-off-by: Gabriel Filion <lelutin@gmail.com>