If you want to take diff of the whole project then these commands can be useful :
For SVN user :
svn diff -r20131:22829 http://svn.openmrs.org/openmrs-modules/feedback/
20131 : Revision to start from
22829 : Revision to end from
http://svn.openmrs.org/openmrs-modules/feedback/ : Link to the SVN .
For Git User :
git log ‘HEAD@{24-05-2011}’..’HEAD@{22-08-2011}’ -p > openmrs-gsoc2011-gauravpaliwal.diff
24-05-2011 : Start Date
22-08-2011 : End Date
openmrs-gsoc2011-gauravpaliwal.diff : File to redirect the output