`ipfs add -w` with stdin input

I wonder what `ipfs add -w` should do when passed stdin input.

Currently it works and ouput a hash, but I wonder what can be done with the hash and where the content passed through stdin went.

For example the following tests work when added at the end of t0040-add-and-cat.sh:

```
test_expect_success "'ipfs add -w' with stdin input succeeds" '
echo "Hello Worlds!" | ipfs add -w >actual
'

test_expect_success "ipfs add -w output looks good" '
HASH="QmSDqa1nBwj2Vstyfqii4SdUbYVVb7cBbkFq78DpsLj5wf" &&
echo "added $HASH " >expected &&
test_cmp expected actual
'
```