release: start subcommand takes an optional base as well
release: start subcommand takes an optional base as well

Signed-off-by: Stefan Naewe <90f89c2062aca940dadf79728a878f6465615f33@googlemail.com>
yonglehou/gitflow-1,oddnoc/gitflow,JelteF/gitflow,dhilgarth/gitflow