Handle case where plan file is at the root

[chefsalim]

This change fixes a couple of cases where the Builder github hook does not kick off builds as expected when there is no explicit .bldr.toml file:

1. If the plan.sh is at the root of the repo, or
2. If the plan.sh is in a habitat subfolder, and the change made is above that folder (eg, in the parent path).

Signed-off-by: Salim Alam salam@chef.io

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@thesentinels approve

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