worktree: detect from secondary worktree if main worktree is bare

When extensions.worktreeConfig is true and the main worktree is
bare -- that is, its config.worktree file contains core.bare=true
-- commands run from secondary worktrees incorrectly see the main
worktree as not bare. As such, those commands incorrectly think
that the repository's default branch (typically "main" or
"master") is checked out in the bare repository even though it's
not. This makes it impossible, for instance, to checkout or delete
the default branch from a secondary worktree, among other
shortcomings.

This problem occurs because, when extensions.worktreeConfig is
true, commands run in secondary worktrees only consult
$commondir/config and $commondir/worktrees/<id>/config.worktree,
thus they never see the main worktree's core.bare=true setting in
$commondir/config.worktree.

Fix this problem by consulting the main worktree's config.worktree
file when checking whether it is bare. (This extra work is
performed only when running from a secondary worktree.)

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Olga Pilipenco <olga.pilipenco@shopify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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