Take commit hash from CircleCI

Similar to the support of Jenkins commit hash. Fixes the same problem as
fixed in 73d1439:

> 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.

source/ci_source/providers/Circle.ts

@@ -96,4 +96,8 @@ export class Circle implements CISource {
   get ciRunURL() {
     return this.env["CIRCLE_BUILD_URL"]
   }
+
+  get commitHash(): string {
+    return this.env.CIRCLE_SHA1
+  }
 }

source/ci_source/providers/_tests/_circle.test.ts

@@ -97,3 +97,19 @@ describe(".repoSlug", () => {
     expect(circle.repoSlug).toEqual("artsy/eigen")
   })
 })
+
+describe("commit hash", () => {
+  it("returns correct commit hash when present", () => {
+    const env = {
+      ...correctEnv,
+      CIRCLE_SHA1: "1234abc",
+    }
+    const circle = new Circle(env)
+    expect(circle.commitHash).toEqual("1234abc")
+  })
+
+  it("returns no commit hash when not present", () => {
+    const circle = new Circle(correctEnv)
+    expect(circle.commitHash).toBeUndefined()
+  })
+})