[ALUG] A rather more specific git question

OK, I have set up a local git repository on my machine and have checked out a mychanges branch so I can add local changes. Is there any git way of comparing the files in the git repository with files in a corresponding (but with *many* more files in) tree somewhere else on my system? I.e. I want to diff each file in the git repository with a file in a similar tree. I think all the git-diff commands just compare different branches within a git repository don't they? -- Chris Green