Take commit hash from CircleCI

[valscion]

Similar to the support of Jenkins commit hash:

danger-js/source/ci_source/providers/Jenkins.ts

Lines 85 to 87 in f58ee85

	get commitHash(): string {
	return this.env.GIT_COMMIT
	}

danger-js/source/ci_source/providers/_tests/_jenkins.test.ts

Lines 113 to 127 in f58ee85

	describe("commit hash", () => {
	it("returns correct commit hash when present", () => {
	const env = {
	...envs.ghprb,
	GIT_COMMIT: "1234abc",
	}
	const jenkins = new Jenkins(env)
	expect(jenkins.commitHash).toEqual("1234abc")
	})
	it("returns no commit hash when not present", () => {
	const jenkins = new Jenkins(envs.ghprb)
	expect(jenkins.commitHash).toBeUndefined()
	})
	})

Fixes the same problem as fixed in #905 but for CircleCI:

Danger doesn't currently have a way to understand on which commit is
running. To get the current commit it asks to the platform the list of
commits on the PR and assumes that is running on the last one. This
creates few problems when you push a new commit before Danger runs on
the CI for the previous commit, because it will assume is running on
the last one, but that is actually not true, and that means that both
the status update and the commit hash in the Danger comment signature
will be wrong.

From the CircleCI docs, CIRCLE_SHA1 should be the correct environment variable: https://circleci.com/docs/2.0/env-vars/#built-in-environment-variables

CIRCLE_SHA1 — String — The SHA1 hash of the last commit of the current build.

[danger-public]

	Warnings
⚠️	These providers are missing from the README: bamboo

Generated by 🚫 dangerJS against bf3e021

[orta]

Cool, this works for me - thanks!

[peril-staging]

Thanks for the PR @valscion.

This PR has been shipped in v10.1.1 - CHANGELOG.