worktree: teach `list --porcelain` to annotate locked worktree

worktree: teach `list --porcelain` to annotate locked worktree

Commit c57b3367be (worktree: teach `list` to annotate locked worktree,
2020-10-11) taught "git worktree list" to annotate locked worktrees by
appending "locked" text to its output, however, this is not listed in
the --porcelain format.

Teach "list --porcelain" to do the same and add a "locked" attribute
followed by its reason, thus making both default and porcelain format
consistent. If the locked reason is not available then only "locked"
is shown.

The output of the "git worktree list --porcelain" becomes like so:

    $ git worktree list --porcelain
    ...
    worktree /path/to/locked
    HEAD 123abcdea123abcd123acbd123acbda123abcd12
    detached
    locked

    worktree /path/to/locked-with-reason
    HEAD abc123abc123abc123abc123abc123abc123abc1
    detached
    locked reason why it is locked
    ...

In porcelain mode, if the lock reason contains special characters
such as newlines, they are escaped with backslashes and the entire
reason is enclosed in double quotes. For example:

   $ git worktree list --porcelain
   ...
   locked "worktree's path mounted in\nremovable device"
   ...

Furthermore, let's update the documentation to state that some
attributes in the porcelain format might be listed alone or together
with its value depending whether the value is available or not. Thus
documenting the case of the new "locked" attribute.

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Rafael Silva <rafaeloliveira.cs@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>