Q:

Is there something similar to diff --show-c-function in git-diff? 

I've been working on a file in my git repository.   If I have a copy of the original git version of the file, I can run diff --show-c-function to get a comparison of the 2, files, where (in theory) it shows the whole of any function which has been changed.   This is really useful for working out what I did specifically to a file, in working out the comment I'd need.   
However in git-diff, this option is missing.   Is there some way to make this work (or am I stuck having to get 2 copies of the file before I can commit, where the git-diff response is slightly complicated)? 

A:

git help diff shows this option:
-W, --function-context
        Show whole surrounding functions of changes. 

So, it looks like git diff -W should do what you want.