sparse-checkout: fix OOM error with mixed patterns

sparse-checkout: fix OOM error with mixed patterns

Add a test to t1091-sparse-checkout-builtin.sh that would result in an
infinite loop and out-of-memory error before this change. The issue
relies on having non-cone-mode patterns while trying to modify the
patterns in cone-mode.

The fix is simple, allowing us to break from the loop when the input
path does not contain a slash, as the "dir" pattern we added does not.

This is only a fix to the critical out-of-memory error. A better
response to such a strange state will follow in a later change.

Reported-by: Calbabreaker <calbabreaker@gmail.com>
Helped-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>