Test chart upgrades

[jlegrone]

What this PR does / why we need it:
This PR enables testing in-place upgrades of charts that have non-breaking changes according to the semver spec.

The desired behavior of this PR and further discussion can be found here: #25 (comment)

Which issue this PR fixes

closes #25
closes #19
closes #99
closes #100

Special notes for your reviewer:

Some TODOs:

• Handle uncommitted changes in git repositories
• Handle not being in a git repository
• Add a config flag to opt out of upgrade testing
• Consider the need for more tests (PR Add Helm/Kubernetes integration tests #120)
• Change the Git interface to a "SCM" interface to avoid further tight coupling with git? (out of scope for this PR)

Known caveats:

1. Only works with git
2. If a chart has direct dependencies outside of its parent directory, helm dependency build may fail or not produce the same result as the previous revision

Run integration tests with go test -cover -tags=integration ./... (see #120)

[jlegrone]

@unguiculus I'd appreciate a first pass when you have a chance!

[unguiculus]

Thanks. Please give me some time to review and test this. From quickly skimming over it: Have you thought about installing the previous version from the repo instead of checking out the merge base? Doing so, you assume the merge base reflects the previous release. This doesn't necessarily have to be the case.

[jlegrone]

I did consider pulling the previous published revision, but I think there are a couple problems with that:

1. We don't necessarily know where the charts have been published -- I suppose we could handle this with a new configuration flag.
2. Not all teams publish their charts to a registry. It is a common use case to version a chart alongside your application source code and run helm commands using the local chart path.

[unguiculus]

LGTM but @scottrigby should also approve.

[scottrigby]

@jlegrone @unguiculus I buried my comment in a resolved thread #103 (comment) - this looks good to me 👍