sparse-checkout: detect short patterns

sparse-checkout: detect short patterns

In cone mode, the shortest pattern the sparse-checkout command will
write into the sparse-checkout file is "/*". This is handled carefully
in add_pattern_to_hashsets(), so warn if any other pattern is this
short. This will assist future pattern checks by allowing us to assume
there are at least three characters in the pattern.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>