Add 'tag -f' support.

Add a new flag, -f/--force, to bup tag. By analogy with 'git tag',
'bup tag -f foo commit' will create tag *foo* even if a tag with that
name already exists; the existing tag will be silently overwritten.

By analogy with 'rm -f', add 'bup tag -d -f'. All this does is
suppress the error when you attempt to 'bup tag -d' a tag that doesn't
exist; it is primarily for use in scripts that want to delete a tag
that may or may not exist in the first place without 'bup tag'
reporting a failure.

Signed-off-by: Ben Kelly <btk@google.com>
[rlb@defaultvalue.org: add missing "tag" and wrap text in bup-tag.md;
 put "-d" first in tag-cmd.py optspec line and bup-tag.md; adjust
 commit message.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
tag-cmd.py: add "FIXME: review for safe writes.".

Signed-off-by: Rob Browning <rlb@defaultvalue.org>