How do I diff the same file between two different commits on the same branch?

Question 1

In Git, how could I compare the same file between two different commits (not contiguous) on the same branch (master for example)?

I'm searching for a compare feature like the one in Visual SourceSafe (VSS) or Team Foundation Server (TFS).
Is it possible in Git?

Question 2

From the git-diff manpage:

git diff [--options] <commit> <commit> [--] [<path>...]

For instance, to see the difference for a file "main.c" between now and two commits back, here are three equivalent commands:

$ git diff HEAD^^ HEAD main.c
$ git diff HEAD^^..HEAD -- main.c
$ git diff HEAD~2 HEAD -- main.c

Question