GeezerVison

Fixes #268.

GNU diff keeps two changes in one hunk as long as at most 2*N unchanged lines separate them (N = context size). We were splitting earlier: after N lines in unified mode and N+1 in context mode. With -U1 on the shopping files from the issue, that gave two hunks, the second one without its leading context line.

Both make_diff functions now start a new hunk only when the gap is larger than 2*N. Context lines in between already go through context_queue, so they now get added to the current hunk.

Added test_hunk_merging to unified_diff.rs and context_diff.rs (gap of 2*N merges, 2*N+1 splits; the expected output comes from GNU diff 3.10). I also ran a few thousand random small file pairs with -U0..4 and -C0..4. The output still applies with patch, and no two hunks touch or overlap anymore. Before this change, about 840 of 3000 runs had adjacent hunks. cargo test, cargo clippy -- -D warnings and cargo fmt --check pass.