Getting the deps for a single commit is overly complicated

I have been using until now git-deps 0.1.0-15-gbd9ed69.
With that `git deps commit` would get the dependencies just for `commit`.

That changed with

```
commit d601e35f6ea42eb6110e3d1449d03f16a1da3f10
Author: Adam Spiers
Date: Thu May 26 00:28:25 2016 +0100

allow passing a revision range to CLI or web UI
```

With that `git deps commit` gets now the dependencies for `commit` and *all* older commits.
To get now the deps just for `commit` one has to use the unwieldy:
`git deps commit^..commit`

Start at the `git deps` CLI handling of revision arguments and compare how a single revision and a range are interpreted. Reproduce `git deps commit` versus `git deps commit^..commit`; done means the single-commit form returns dependencies only for that commit while explicit ranges continue to work.

Written by the indexing model from the issue text.