mv: from in-cone to out-of-cone

Originally, moving an in-cone <source> to an out-of-cone <destination>
was not possible, mainly because such <destination> is a directory that
is not present in the working tree.

Change the behavior so that we can move an in-cone <source> to
out-of-cone <destination> when --sparse is supplied.

Notice that <destination> can also be an out-of-cone file path, rather
than a directory.

Such <source> can be either clean or dirty, and moving it results in
different behaviors:

A clean move should move <source> to <destination> in the index (do
*not* create <destination> in the worktree), then delete <source> from
the worktree.

A dirty move should move the <source> to the <destination>, both in the
working tree and the index, but should *not* remove the resulted path
from the working tree and should *not* turn on its CE_SKIP_WORKTREE bit.

Optional reading
================

We are strict about cone mode when <destination> is a file path.
The reason is that some of the previous tests that use no-cone mode in
t7002 are keep breaking, mainly because the `dst_mode = SPARSE;` line
added in this patch.

Most features developed in both "from-out-to-in" and "from-in-to-out"
only care about cone mode situation, as no-cone mode is becoming
irrelevant. And because assigning `SPARSE` to `dst_mode` when the
repo is in no-cone mode causes miscellaneous bugs, we should just leave
this new functionality to be exclusive cone mode and save some time.

Helped-by: Derrick Stolee <derrickstolee@github.com>
Helped-by: Victoria Dye <vdye@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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