Behave correctly on non-Go repositories

[bradleyfalzon]

As a user, I should be able to enable GopherCI on all my repositories, and GopherCI should either not appear in the GitHub Status API at all, or it should never fail.

Currently it fails because on an empty repository (a repository without any go files with a package declaration), go get fails.

root@916823289afa:/go/src/github.com/bf-test/gopherci-itest# go get -t ./...
# cd /go/src/github.com/bf-test/gopherci-itest; git pull --ff-only
You are not currently on a branch. Please specify which
branch you want to merge with. See git-pull(1) for details.

    git pull <remote> <branch>

package github.com/bf-test/gopherci-itest/...: exit status 1

If there's a .go file with a package declaration anywhere, this isn't an issue.

Ideally, GopherCI would never be executed at all, but more realistically, we may need to close the repository before we know whether there's Go files there at all. By the time we clone, the status API has already been set.

It may be possible to use GitHub API to check the repositories file list or similar, but this is GitHub specific. It may have its own inconsistencies (we need to ensure it reports the file list at that specific commit - the head ref in this case). This would save bandwidth, not downloading repositories that can't be checked, but we still need to fix the underlying issue for a VCS that isn't GitHub.

EDIT: So I think the approach I'll take is to use the GitHub API (another API call for a PR, but a push contains changed/added/removed files) to determine if Go files were added/changed/removed. If none, we won't touch the status API at all. This would be GitHub specific, so docs for Analyser should be updated to require repositories that have Go code, or handle the scenario more gracefully).

[bradleyfalzon]

And importantly, customers should not get billed for non Go projects, even if we had to queue and clone them.

[bradleyfalzon]

Related to #73 because we can use the same mechanism to determine if repo is public or private.