yes: improve efficiency when all args aren't buffered

yes: improve efficiency when all args aren't buffered

* src/yes.c (main): Even when the internal buffer isn't large enough,
output what we've buffered already, and interate over the rest.
This improves the performance in the edge case where there are
many small arguments that overflow the buffer.
* tests/misc/yes.sh: Add a test case for the many small arguments case.