To show a history of changes to a particular file, you can use git log:
git log -p -- path/to/file
The -p tells it to show the diff between each revision and its parent. To get a cumulative diff between two revisions, take the ID of the two revisions, and pass them to git diff:
git diff abc123 def456 -- path/to/file.
Where abc123 and def456 are the revision IDs.