submodule: unset core.worktree if no working tree is present

submodule: unset core.worktree if no working tree is present

When a submodules work tree is removed, we should unset its core.worktree
setting as the worktree is no longer present. This is not just in line
with the conceptual view of submodules, but it fixes an inconvenience
for looking at submodules that are not checked out:

    git clone --recurse-submodules git://github.com/git/git && cd git &&
    git checkout --recurse-submodules v2.13.0
    git -C .git/modules/sha1collisiondetection log
    fatal: cannot chdir to '../../../sha1collisiondetection': \
        No such file or directory

With this patch applied, the final call to git log works instead of dying
in its setup, as the checkout will unset the core.worktree setting such
that following log will be run in a bare repository.

This patch covers all commands that are in the unpack machinery, i.e.
checkout, read-tree, reset. A follow up patch will address
"git submodule deinit", which will also make use of the new function
submodule_unset_core_worktree(), which is why we expose it in this patch.

This patch was authored as 4fa4f90ccd (submodule: unset core.worktree if
no working tree is present, 2018-06-12), which was reverted as part of
f178c13fda (Revert "Merge branch 'sb/submodule-core-worktree'",
2018-09-07). The revert was needed as the nearby commit e98317508c
(submodule: ensure core.worktree is set after update, 2018-06-18) is
faulty and at the time of 7e25437d35 (Merge branch
'sb/submodule-core-worktree', 2018-07-18) we could not revert the faulty
commit only, as they were depending on each other: If core.worktree is
unset, we have to have ways to ensure that it is set again once
the working tree reappears again.

Now that 4d6d6ef1fc (Merge branch 'sb/submodule-update-in-c', 2018-09-17),
specifically 74d4731da1 (submodule--helper: replace
connect-gitdir-workingtree by ensure-core-worktree, 2018-08-13) is
present, we already check and ensure core.worktree is set when
populating a new work tree, such that we can re-introduce the commits
that unset core.worktree when removing the worktree.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>