I don't know if git has concept of revision.
I'd like to diff current working copy with older version of it(not neccesarily a last commit)
Would be great if it could be done inside emacs.
I've no idea about doing this inside emacs, but:
git diff rev1..rev2
will do what you want for the whole repository. You can then add to that:
git diff rev1>..rev2 path
path can be an absolute path to a file, or a path to a directory, so you can diff subtrees.
If you're looking to diff the working copy - just specify revision 1:
git diff rev1 path
The revision codes can be special names, e.g. HEAD^..HEAD means last to current, or their sha values from the logs.
来源：https://stackoverflow.com/questions/11341546/how-to-diff-a-file-with-specific-revision-in-git