worktree: add --quiet option

worktree: add --quiet option

Add the '--quiet' option to git worktree, as for the other git
commands. 'add' is the only command affected by it since all other
commands, except 'list', are currently silent by default.

[jc: appiled trivial fix-up to keep the tests from touching outside
the scratch area]

Helped-by: Martin Ågren <martin.agren@gmail.com>
Helped-by: Duy Nguyen <pclouds@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>