checkout: prevent losing staged changes with --merge

checkout: prevent losing staged changes with --merge

When --merge is specified, we may need to do a real merge (instead of
three-way tree unpacking), the steps are best seen in git-checkout.sh
version before it's removed:

    # Match the index to the working tree, and do a three-way.
    git diff-files --name-only | git update-index --remove --stdin &&
    work=`git write-tree` &&
    git read-tree $v --reset -u $new || exit

    git merge-recursive $old -- $new $work

    # Do not register the cleanly merged paths in the index yet.
    # this is not a real merge before committing, but just carrying
    # the working tree changes along.
    unmerged=`git ls-files -u`
    git read-tree $v --reset $new
    case "$unmerged" in
    '')     ;;
    *)
            (
                    z40=0000000000000000000000000000000000000000
                    echo "$unmerged" |
                    sed -e 's/^[0-7]* [0-9a-f]* /'"0 $z40 /"
                    echo "$unmerged"
            ) | git update-index --index-info
            ;;
    esac

Notice the last 'read-tree --reset' step. We restore worktree back to
'new' tree after worktree's messed up by merge-recursive. If there are
staged changes before this whole command sequence is executed, they
are lost because they are unlikely part of the 'new' tree to be
restored.

There is no easy way to fix this. Elijah may have something up his
sleeves [1], but until then, check if there are staged changes and
refuse to run and lose them. The user would need to do "git reset" to
continue in this case.

A note about the test update. 'checkout -m' in that test will fail
because a deletion is staged. This 'checkout -m' was previously needed
to verify quietness behavior of unpack-trees. But a different check
has been put in place in the last patch. We can safely drop
'checkout -m' now.

[1] CABPp-BFoL_U=bzON4SEMaQSKU2TKwnOgNqjt5MUaOejTKGUJxw@mail.gmail.com

Reported-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>