small fix for git-diff man page

A small pull request.

https://github.com/git/git/pull/580

Git diff can work with a tree in the form git diff tree..tree too, only the form git diff commit...commit can't accept a tree instead of a commit.

Also added useful example about using a tree with git diff.

But what is strange. I changed only one asciidoc file. But my pull request can not pass an integration checks. «Git.git (Documentation)» is successfully passed, but «git.git (linux-gcc)», «git.git», «continuous-integration/travis-ci/pr»  and I don’t see reason for it.