Replace svn__atoui64 with svn_cstring_atoui64.
Replace svn__atoui64 with svn_cstring_atoui64.

The svn_cstring_atoui64() returns svn_error_t * on failure and thus is
more safe to use.

satori/git-svn-fast-import,satori/git-svn-fast-import,satori/git-svn-fast-import