completion: list existing working trees for 'git worktree' subcommands

completion: list existing working trees for 'git worktree' subcommands

Complete the paths of existing working trees for 'git worktree's
'move', 'remove', 'lock', and 'unlock' subcommands.

Note that 'git worktree list --porcelain' shows absolute paths, so for
simplicity's sake we'll complete full absolute paths as well (as
opposed to turning them into relative paths by finding common leading
directories between $PWD and the working tree's path and removing
them, risking trouble with symbolic links or Windows drive letters; or
completing them one path component at a time).

Never list the path of the main working tree, as it cannot be moved,
removed, locked, or unlocked.

Ideally we would only list unlocked working trees for the 'move',
'remove', and 'lock' subcommands, and only locked ones for 'unlock'.
Alas, 'git worktree list --porcelain' doesn't indicate which working
trees are locked, so for now we'll complete the paths of all existing
working trees.

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