worktree: define subcommand -h in terms of command -h

worktree: define subcommand -h in terms of command -h

Avoid repeating the "-h" output for the "git worktree" command, and
instead define the usage of each subcommand with macros, so that the
"-h" output for the command itself can re-use those definitions. See
[1], [2] and [3] for prior art using the same pattern.

1. b25b727494f (builtin/multi-pack-index.c: define common usage with a
   macro, 2021-03-30)
2. 8757b35d443 (commit-graph: define common usage with a macro,
   2021-08-23)
3. 1e91d3faf6c (reflog: move "usage" variables and use macros,
   2022-03-17)

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