mv: check if <destination> exists in index to handle overwriting

mv: check if <destination> exists in index to handle overwriting

Originally, moving a sparse file into cone can result in unwarned
overwrite of existing entry. The expected behavior is that if the
<destination> exists in the entry, user should be prompted to supply
a [-f|--force] to carry out the operation, or the operation should
fail.

Add a check mechanism to do that.

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