grep: avoid separators for adjacent context groups

[wondr-wclabs]

Fixes #24.

When a later match pulls in before-context that starts immediately after the last printed line, the output groups are adjacent and should be merged. The old separator check compared the match line directly with the last printed line before draining before-context, so it inserted -- even though the next emitted context line closed the gap.

This uses the first buffered context line that will actually be printed as the group start, falling back to the match line when there is no pending context.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing wondr-wclabs:codex/context-adjacent-groups (ee6ef1e) with main (f4798cb)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩