The benchmark in this commit speeds up from ~360us to ~210us.

Data generated using the following command run on rust-lang/rust#c102c5cf:

    $ for sha in `git rev-list HEAD -- .gitignore`; do git show ${sha}:.gitignore >> gitignores.txt; echo '!!!' >> gitignores.txt; done

This version allocations a bit more memory up front but is faster in all
benchmarks, sometimes significantly faster.

group                                                 after                  before
-----                                                 -----                  ------
10 equal items                                        1.00   171.8±11.75ns   1.09    187.6±7.73ns
10 non-equal items                                    1.00  1150.8±36.11ns   1.08  1242.8±36.13ns
100 equal items                                       1.00   296.0±10.78ns   1.24   366.0±13.09ns
100 non-equal items                                   1.00     29.2±1.57µs   1.15     33.5±1.38µs
1000 equal items                                      1.00  1354.9±36.02ns   1.50      2.0±0.06µs
1000 non-equal items                                  1.00  1868.1±68.60µs   1.24      2.3±0.07ms
1024 byte string, last 256 different                  1.00    143.9±3.67µs   1.47    212.1±7.86µs
diff::chars on gitignore files from rust-lang/rust    1.00     68.6±1.99ms   1.46    100.5±2.87ms
diff::lines on gitignore files from rust-lang/rust    1.00      5.9±0.15ms   3.69     21.7±0.47ms
empty                                                 1.00    109.1±4.98ns   1.14    124.2±4.88ns

group                                                 after                  before
-----                                                 -----                  ------
10 equal items                                        1.00    113.1±3.09ns   1.50    169.6±5.33ns
10 non-equal items                                    1.00   498.5±19.32ns   2.28  1137.9±43.20ns
100 equal items                                       1.00    257.9±8.30ns   1.14   295.2±11.59ns
100 non-equal items                                   1.00     17.4±0.40µs   1.66     28.8±1.02µs
1000 equal items                                      1.12  1534.4±45.30ns   1.00  1363.9±41.31ns
1000 non-equal items                                  1.08      2.0±0.08ms   1.00  1854.9±64.81µs
1024 byte string, last 256 different                  1.00    101.2±3.13µs   1.43    144.6±4.20µs
diff::chars on gitignore files from rust-lang/rust    1.00     65.9±2.54ms   1.04     68.7±2.67ms
diff::lines on gitignore files from rust-lang/rust    1.00      4.9±0.18ms   1.20      5.9±0.15ms
empty                                                 1.00     58.1±2.91ns   1.85    107.4±3.85ns