dog: allow snapshot tags which begin with decimal number

Current dog command doesn't accept snapshot tags which begin with
decimal number, e.g. 123abc. This behavior comes from invalid usage of
strtol(). This patch fixes this problem.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>