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>