emits a spurious `--` context separator between adjacent groups

[sylvestre]

With before-context (-B/-C), uu_grep prints a -- group separator between two output groups even when they are adjacent — i.e. the next group's context begins on the line immediately after the last line already printed. GNU only prints -- when there is a real gap (one or more skipped lines) between groups.

Found by the differential fuzzer (fuzz_grep).

Input is five lines: a, b, empty, empty, c. Pattern . matches the three non-empty lines.

Rust (incorrect)

$ printf 'a\nb\n\n\nc\n' | ./target/release/grep -e '.' -B 2
a
b
--


c
# Exit code: 0

GNU (correct)

$ printf 'a\nb\n\n\nc\n' | LC_ALL=C /usr/bin/grep -e '.' -B 2
a
b


c
# Exit code: 0

The first group ends at line 2 (b). The match on line 5 (c) pulls in before-context lines 3 and 4, so the second group starts at line 3 — immediately after line 2. The groups touch, so no separator belongs there: GNU merges them, uu_grep inserts --.