git-cvsimport-3: actually invoke git-branch

The command called if a branch is renamed is "branch -m".  This should
clearly by "git branch -m".

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: fix moving tags

The command "git tag -f $tag $changed" moves $tag to point at commit
$changed but $changed is the new name for the tag.  Fix this by
reordering the arguments to "git tag".

Also add a call to "git update-ref -d" to remove the old tag.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: use for-each-ref not "branch -l"

git-branch is a porcelain command, switch to the plumbing command
git-for-each-ref so that we're not relying on the output format of
git-branch.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: use for-each-ref not "tag -l"

git-tag is a porcelain command, switch to the plumbing command
git-for-each-ref so that we're not relying on the output format of
git-tag.

Signed-off-by: John Keeping <john@keeping.me.uk>