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>

dir.c

@@ -142,7 +142,7 @@ static int add_excludes_from_file_1(const char *fname,
 		return 0;
 	}
 	buf = xmalloc(size+1);
-	if (read(fd, buf, size) != size)
+	if (read_in_full(fd, buf, size) != size)
 		goto err;
 	close(fd);