generate-cmdlist.sh: don't parse command-list.txt thrice

generate-cmdlist.sh: don't parse command-list.txt thrice

Change the "define_categories()" and "define_category_names()" functions
to take the already-parsed output of "category_list()" as an argument,
which brings our number of passes over "command-list.txt" from three
to two.

Then have "category_list()" itself take the output of "command_list()"
as an argument, bringing the number of times we parse the file to one.

Compared to the pre-image this speeds us up quite a bit:

    $ git show HEAD~:generate-cmdlist.sh >generate-cmdlist.sh.old
    $ hyperfine --warmup 10 -L v ,.old 'sh generate-cmdlist.sh{v} command-list.txt'
    Benchmark #1: sh generate-cmdlist.sh command-list.txt
      Time (mean ± σ):      22.9 ms ±   0.3 ms    [User: 15.8 ms, System: 9.6 ms]
      Range (min … max):    22.5 ms …  24.0 ms    125 runs

    Benchmark #2: sh generate-cmdlist.sh.old command-list.txt
      Time (mean ± σ):      30.1 ms ±   0.4 ms    [User: 24.4 ms, System: 17.5 ms]
      Range (min … max):    29.5 ms …  32.3 ms    96 runs

    Summary
      'sh generate-cmdlist.sh command-list.txt' ran
        1.32 ± 0.02 times faster than 'sh generate-cmdlist.sh.old command-list.txt'

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>