Fix misc/check-no-diff on macOS.

`diff` on macOS does not support the `--left-column` flag.

Use `sdiff` (side-by-side diff) instead, which is part of diffutils.

Author: dan-zheng

misc/check-no-diff

@@ -11,7 +11,7 @@
 # non-zero error status.
 
 tmpdiff=$(mktemp)
-diff --left-column -y $1 $2 > $tmpdiff \
+sdiff --left-column $1 $2 > $tmpdiff \
        && echo OK || (cat $tmpdiff; false)
 status=$?