Don't fetch a diff on first commit

[bradleyfalzon]

Currently, the first commit has the latest commit with a relative
reference of number of commits being pushed. For example, if the
first event has just 1 commit at sha abcdef, commitFrom is abcdef~1
and commitTo is abcdef.

We use relative references to ensure we can always fetch the diff
even if the oldest commit has been removed (via push). But the first
commit doesn't appear to have a valid diff from any GitHub APIs so
to indicate this, we'll set commitFrom to blank.

When it comes to fetching the diff via the VCSReader interface, if
commitFrom is blank, return a nil io.ReadCloser and nil error.

The frontends already know how to handle lack of a diff, it just
occurs even when there's no error now.

This also relates to #79, which was handling fetching diffs when
the before ref no longer exists, which is the case when a branch
tracks a new tree or a new repository is created. I couldn't
combine the logic much further, because GitHub has no indication
when the commit follows a new tree (the before ref is from the
previous tree, whereas in a new repository it's all 0s).

So I wasn't able to combine any logic between this and #79, but
the integration test was renamed from new-go-repository to new-tree
to better reflect what it was doing.

Fixes #96.
Relates to #79.

[coveralls]

Coverage increased (+0.05%) to 60.327% when pulling 788e00a on first-commit into 9e35dc9 on master.