Print version diff for rust and cargo since last rustup.sh invocaction

I've been using this patch on my local rustup.sh for sometime and quite like it as a way to see what has changed since my last update.

The prevailing style in this file seems to be 8 space indent in condition bodies, 4 elsewhere.
Though I'm unfamiliar with that style I tried to match it.

Also I'm no bash guru so please give a review before accepting :sweat_smile: 

The output looks like this:
```
... 

install: verifying installed binaries are executable

    Rust is ready to roll.

    Went from version:
    rustc 0.13.0-nightly (1652a1f2c 2014-10-28 22:11:56 +0000)
    to
    rustc 0.13.0-nightly (15dd90b64 2014-10-30 00:27:02 +0000)
    See diff here: https://github.com/rust-lang/rust/compare/1652a1f2c...15dd90b64

...

install: verifying installed binaries are executable

    Cargo is ready to roll.

    Went from version:
    cargo 0.0.1-pre-nightly (947a62b 2014-10-28 19:59:49 +0000)
    to
    cargo 0.0.1-pre-nightly (041d14e 2014-10-29 19:32:55 +0000)
    See diff here: https://github.com/rust-lang/cargo/compare/947a62b...041d14e
```