update_linked_gitdir writes relative path to .git/worktrees/<id>/gitdir

I noticed that when update_linked_gitdir chooses to update
.git/worktrees/<id>/gitdir, the path it writes is relative, at least
under some circumstances.  This contradicts the gitrepository-layout
man page, which says:

worktrees/<id>/gitdir::
        A text file containing the absolute path back to the .git file
        that points to here.

IIUC, this behavior defeats one of the three safeguards that is
supposed to prevent "git worktree prune" from pruning information for
worktrees that still exist.

A simple script to reproduce:

#!/bin/bash
set -e -x
rm -rf repo worktree2
git init repo
cd repo
touch foo
git add foo
git commit -m 'dummy commit'
git worktree add ../worktree2 -b branch2
cat .git/worktrees/worktree2/gitdir
touch -d '2 days ago' .git/worktrees/worktree2/gitdir
(cd ../worktree2 && git status)
cat .git/worktrees/worktree2/gitdir

Trying this on master as of earlier today (ff4ea60), I get:

[...]
/PATH/REDACTED/worktree2/.git
[...]
.git

Matt