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>
diff --git a/refs.c b/refs.c
index 5205745..2b69e1e 100644
--- a/refs.c
+++ b/refs.c
@@ -284,7 +284,7 @@
 		fd = open(path, O_RDONLY);
 		if (fd < 0)
 			return NULL;
-		len = read(fd, buffer, sizeof(buffer)-1);
+		len = read_in_full(fd, buffer, sizeof(buffer)-1);
 		close(fd);
 
 		/*