completion: simplify completing 'git worktree' subcommands and options

completion: simplify completing 'git worktree' subcommands and options

The completion function for 'git worktree' uses separate but very
similar case arms to complete --options for each subcommand.

Combine these into a single case arm to avoid repetition.

Note that after this change we won't complete 'git worktree remove's
'--force' option, but that is consistent with our general stance on
not offering '--force', as it should be used with care.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>