diff: optimize allocation of histogram entries for unique words

[yuja]

group                             new                     old
-----                             ---                     ---
bench_diff_git_git_read_tree_c    1.00     34.5±0.26µs    1.32     45.7±0.11µs
bench_diff_lines/modified/10k     1.00     28.2±0.10ms    1.19     33.5±0.69ms
bench_diff_lines/modified/1k      1.00      2.6±0.01ms    1.15      3.0±0.01ms
bench_diff_lines/reversed/10k     1.00     21.5±0.22ms    1.08     23.3±0.18ms
bench_diff_lines/reversed/1k      1.00   364.8±11.96µs    1.22    445.1±8.99µs
bench_diff_lines/unchanged/10k    1.00  1761.3±13.85µs    1.66      2.9±0.07ms
bench_diff_lines/unchanged/1k     1.00    163.6±1.25µs    1.47    240.7±2.72µs

% hyperfine --sort command --warmup 3 --runs 5 -L bin jj-0,jj-1 \
  'target/release-with-debug/{bin} --ignore-working-copy \
  file annotate lib/src/revset.rs'
Benchmark 1: target/release-with-debug/jj-0 ..
  Time (mean ± σ):      1.144 s ±  0.011 s    [User: 1.088 s, System: 0.053 s]
  Range (min … max):    1.131 s …  1.159 s    5 runs

Benchmark 2: target/release-with-debug/jj-1 ..
  Time (mean ± σ):      1.026 s ±  0.008 s    [User: 0.975 s, System: 0.048 s]
  Range (min … max):    1.015 s …  1.035 s    5 runs

Checklist

If applicable:

• [ ] I have updated CHANGELOG.md
• [ ] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)
• [ ] I have added tests to cover my changes