描述
start-commit在开始事务之前执行，通常是用来确定用户是否有提交权限。
If the start-commit hook program returns a nonzero exit
value, the commit is stopped before the commit transaction
is even created, and anything printed to stderr is
marshalled back to the client.
输入参数
传递给你钩子程序的命令行参数，按照顺序是：
Repository path
Authenticated username attempting the commit
(New in Subversion 1.5.) Colon-separated list of
capabilities that a client passes to the server,
including depth,
mergeinfo, and
log-revprops.
普通用户
Access control (e.g., temporarily lock out commits for
some reason).
A means to allow access only from clients that have
certain capabilities.