If you want to get the difference between two branches, say master and branch-name, use the following command:
git diff master..branch-name
If you want that same diff in a patch, because patches are handy, just add the output redirect:
git diff master..branch-name > branch-name.patch
If you need to import that patch into something like Crucible then you'll need to get rid of the a and b prefixes that git adds:
git diff --no-prefix master..branch-name > branch-name.patch