Fix --revision-range=SINGLE_COMMIT on git.

Before this, we would generate a diff with

    git diff SINGLE_COMMIT..CURRENT_HEAD

but this is not what was intended and what we write in the docs.
Now, we use

    git show SINGLE_COMMIT

to really only generate the diff of the given commit.

Things like `rbtools/postreview.py -n --revision-range=master^` work as expected now.