DIR or $GIT_COMMON_DIR depending on the path. For example, in the
linked working tree <code>git rev-parse --git-path HEAD</code> returns
<code>/path/main/.git/worktrees/test-next/HEAD</code> (not
<code>/path/other/test-next/.git/HEAD</code> or <code>/path/main/.git/HEAD</code>) while <code>git
rev-parse --git-path refs/heads/master</code> uses
$GIT_COMMON_DIR and returns <code>/path/main/.git/refs/heads/master</code>,
since refs are shared across all working trees, except refs/bisect and
refs/worktree.</p>
