short i/o: fix calls to read to use xread or read_in_full

We have a number of badly checked read() calls.  Often we are
expecting read() to read exactly the size we requested or fail, this
fails to handle interrupts or short reads.  Add a read_in_full()
providing those semantics.  Otherwise we at a minimum need to check
for EINTR and EAGAIN, where this is appropriate use xread().

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-upload-archive.c

@@ -91,7 +91,7 @@ static void process_input(int child_fd, int band)
 	char buf[16384];
 	ssize_t sz = read(child_fd, buf, sizeof(buf));
 	if (sz < 0) {
-		if (errno != EINTR)
+		if (errno != EAGAIN && errno != EINTR)
 			error_clnt("read error: %s\n", strerror(errno));
 		return;
 	}