Use Git Commit type in GitHub mergedAs edge (#407)

Test plan:
`yarn travis` passes

src/v3/plugins/github/__snapshots__/edges.test.js.snap

@@ -83,8 +83,9 @@ Object {
     "5",
   ],
   "dstParts": Array [
+    "sourcecred",
     "git",
-    "commit",
+    "COMMIT",
     "123",
   ],
   "srcParts": Array [

src/v3/plugins/github/edges.js

@@ -3,11 +3,11 @@
 import {
   type Edge,
   type EdgeAddressT,
-  type NodeAddressT,
   EdgeAddress,
   NodeAddress,
 } from "../../core/graph";
 import * as GithubNode from "./nodes";
+import * as GitNode from "../git/nodes";
 
 export opaque type RawAddress: EdgeAddressT = EdgeAddressT;
 
@@ -65,11 +65,11 @@ export const createEdge = Object.freeze({
   }),
   mergedAs: (
     pull: GithubNode.PullAddress,
-    commitAddress: NodeAddressT /* TODO: Make this a Git commit node address. */
+    commit: GitNode.CommitAddress
   ): Edge => ({
     address: toRaw({type: MERGED_AS_TYPE, pull}),
     src: GithubNode.toRaw(pull),
-    dst: commitAddress,
+    dst: GitNode.toRaw(commit),
   }),
   hasParent: (
     child: GithubNode.ChildAddress,

src/v3/plugins/github/edges.test.js

@@ -1,9 +1,10 @@
 // @flow
 
-import {type EdgeAddressT, NodeAddress, edgeToParts} from "../../core/graph";
+import {type EdgeAddressT, edgeToParts} from "../../core/graph";
 import {createEdge, fromRaw, toRaw} from "./edges";
 import * as GE from "./edges";
 import * as GN from "./nodes";
+import {COMMIT_TYPE} from "../git/nodes";
 
 describe("plugins/github/edges", () => {
   const nodeExamples = {
@@ -45,8 +46,8 @@ describe("plugins/github/edges", () => {
     authors: () =>
       createEdge.authors(nodeExamples.user(), nodeExamples.issue()),
     mergedAs: () => {
-      const commitAddress = NodeAddress.fromParts(["git", "commit", "123"]);
+      const commit = {type: COMMIT_TYPE, hash: "123"};
-      return createEdge.mergedAs(nodeExamples.pull(), commitAddress);
+      return createEdge.mergedAs(nodeExamples.pull(), commit);
     },
     hasParent: () =>
       createEdge.hasParent(nodeExamples.reviewComment(), nodeExamples.review()),