Ensure return value from xread() is always stored into an ssize_t

This patch fixes all calls to xread() where the return value is not
stored into an ssize_t. The patch should not have any effect whatsoever,
other than putting better/more appropriate type names on variables.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.c

@@ -1411,7 +1411,7 @@ static int populate_from_stdin(struct diff_filespec *s)
 #define INCREMENT 1024
 	char *buf;
 	unsigned long size;
-	int got;
+	ssize_t got;
 
 	size = 0;
 	buf = NULL;