Fix add hang when dest is .

Fixes #3960
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
adding test for hanging ADD src .

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
Merge pull request #25 from tiborvass/3960-add-file-hangs

adding test for hanging ADD src .
Merge pull request #5997 from crosbymichael/add-hang

Fix add hang when dest is .