This function is no longer calling the `git diff` command, so this name
doesn't make sense.

This reverts commit 588cd526fd78b3a0e0ec1dfb9fc194ca385de902.

Since each suggestion has its own associated commit, we can't compare a
tree and a single commit. We'll probably have to invoke `git diff` once
for each suggestion.

This will enable us to use the `git diff` command with more than one
suggestion at the same time.

We compare the commit of the suggestion with the new index's tree.

And as I write this, I'm realising that we can't diff multiple
suggestions using the same commit because each suggestion has its own
commit.

Need to work out a different way to do this.

Don't run the `git diff` command in this function, instead return the
new blob created for the right hand side of the comparison.

This will facilitate showing the diffs of multiple suggestions at once.
Maybe. I haven't tried the command yet.

Actually, looking at the man page, it doesn't say we can include more
than one blob pair, so I might need to put all of the suggestion changes
into a single blob and compare the original commit with that.

I want to enable colour diff output. At first, I thought about doing
this by getting the diff colours from the Git config, then outputting
the ANSI escape codes manually with the text.

However, I then thought it would be nice to enable other diff features
like `--word-diff`, and it would be too much trouble to re-implement
that manually, and then I'd have to take word diff colouring into
account.

Decided that the optimal way to get both colours and extra diff
features like word diffing would be to use `git diff` directly under the
hood.

We shell out to `git diff`, and give it the original file as a base, and
a new blob object that we create in the repository on the right side.
This allows us to compare the file using a command in the following
format:

    $ git diff <BASE>:src/server.rs <TEMPORARY_HASH>

Ostensibly, the temporary blob object I create for the right side
version should be freed automatically by `git2`.

Add a new `sugdiff` command to output diffs. This will replace
`sugpatch`. Since we're working with diffs, I think a name with "diff"
sounds more logical. The previous version, "patch" sounds too much like
what "apply" does, because of the Unix `patch` command.

Since this is a Git command, this makes more sense to me.

Forgot to include this so we didn't get extra information about the
cause of the error.

Light documentation for our various functions and types.