sparse-checkout: allow one-character directories in cone mode

In 9e6d3e64 (sparse-checkout: detect short patterns, 2020-01-24), a condition on the minimum length of a cone-mode pattern was introduced. However, this condition was off-by-one. If we have a directory with a single character, say "b", then the command git sparse-checkout set b will correctly add the pattern "/b/" to the sparse-checkout file. When this is interpeted in dir.c, the pattern is "/b" with the PATTERN_FLAG_MUSTBEDIR flag. This string has length two, which satisfies our inclusive inequality (<= 2). The reason for this inequality is that we will start to read the pattern string character-by-character using three char pointers: prev, cur, next. In particular, next is set to the current pattern plus two. The mistake was that next will still be a valid pointer when the pattern length is two, since the string is null-terminated. Make this inequality strict so these patterns work. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>