upload-archive: fix infinite loop on Cygwin

upload-archive: fix infinite loop on Cygwin

On Cygwin, poll() reports POLLIN even for file descriptors that have
reached their end.  This caused git upload-archive to be stuck in an
infinite loop, as it only looked at the POLLIN flag.

In addition to POLLIN, check if read() returned 0, which indicates
end-of-file, and keep looping only as long as at least one of the file
descriptors has input.  This lets the following command finish on its
own when run in a git repository on Cygwin, instead of it getting stuck
after printing all file names:

	$ git archive -v --remote . HEAD >/dev/null

Reported-by: Bob Kagy <bobkagy@gmail.com>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>