Add `git/render.js` with `nodeDescription` (#262)

Test Plan:
Unit tests added, with full coverage of reachable cases.

wchargin-branch: git-render

src/plugins/git/render.js

@@ -0,0 +1,30 @@
+// @flow
+
+import type {Address} from "../../core/address";
+import type {Graph} from "../../core/graph";
+import type {NodeType, SubmoduleCommitPayload} from "./types";
+
+/**
+ * Describe a node provided by the Git plugin.
+ */
+export function nodeDescription(graph: Graph<any, any>, address: Address) {
+  const type: NodeType = (address.type: any);
+  switch (type) {
+    case "COMMIT":
+      return `commit ${address.id}`;
+    case "TREE":
+      return `tree ${address.id}`;
+    case "BLOB":
+      return `blob ${address.id}`;
+    case "SUBMODULE_COMMIT": {
+      const payload: SubmoduleCommitPayload = graph.node(address).payload;
+      return `submodule commit ${payload.hash} in ${payload.url}`;
+    }
+    case "TREE_ENTRY":
+      return `entry ${address.id}`;
+    default:
+      // eslint-disable-next-line no-unused-expressions
+      (type: empty);
+      throw new Error(`unknown type: ${type}`);
+  }
+}

src/plugins/git/render.test.js

@@ -0,0 +1,70 @@
+// @flow
+
+import type {
+  CommitNodePayload,
+  BlobNodePayload,
+  TreeNodePayload,
+  TreeEntryNodePayload,
+  SubmoduleCommitPayload,
+} from "./types";
+import type {Node} from "../../core/graph";
+import {Graph} from "../../core/graph";
+import {_makeAddress, commitAddress} from "./address";
+import {nodeDescription} from "./render";
+import {submoduleCommitId, treeEntryId} from "./types";
+
+describe("nodeDescription", () => {
+  it("describes commits", () => {
+    const node: Node<CommitNodePayload> = {
+      address: commitAddress("cafebabe"),
+      payload: (({}: any): {||}),
+    };
+    expect(nodeDescription(new Graph().addNode(node), node.address)).toEqual(
+      "commit cafebabe"
+    );
+  });
+
+  it("describes trees", () => {
+    const node: Node<TreeNodePayload> = {
+      address: _makeAddress("TREE", "deadbeef"),
+      payload: (({}: any): {||}),
+    };
+    expect(nodeDescription(new Graph().addNode(node), node.address)).toEqual(
+      "tree deadbeef"
+    );
+  });
+
+  it("describes blobs", () => {
+    const node: Node<BlobNodePayload> = {
+      address: _makeAddress("BLOB", "01010101"),
+      payload: (({}: any): {||}),
+    };
+    expect(nodeDescription(new Graph().addNode(node), node.address)).toEqual(
+      "blob 01010101"
+    );
+  });
+
+  it("describes submodule commits", () => {
+    const hash = "15615651";
+    const url = "https://github.com/sourcecred/example-git-submodule.git";
+    const node: Node<SubmoduleCommitPayload> = {
+      address: _makeAddress("SUBMODULE_COMMIT", submoduleCommitId(hash, url)),
+      payload: {hash, url},
+    };
+    expect(nodeDescription(new Graph().addNode(node), node.address)).toEqual(
+      `submodule commit ${hash} in ${url}`
+    );
+  });
+
+  it("describes tree entries", () => {
+    const tree = "76476476323";
+    const name = "healing";
+    const node: Node<TreeEntryNodePayload> = {
+      address: _makeAddress("TREE_ENTRY", treeEntryId(tree, name)),
+      payload: {name},
+    };
+    expect(nodeDescription(new Graph().addNode(node), node.address)).toEqual(
+      `entry ${tree}:${name}`
+    );
+  });
+});