HBASE-11721 jdiff script no longer works as usage instructions indicate

* Add L argument to the curl pull of JDiff to handle following redirects.
* Change how Git repos are cloned to reflect how shallow clones work
  now.
* Change arguments to support BSD sed (i.e. on Mac OS X).
* chmod +x on the script.

Signed-off-by: Jonathan M Hsieh <jmhsieh@apache.org>