Don’t load trees from Git repositories (#730)

Summary:
We currently load trees and then throw them away later, because we don’t
get useful signal from them. We should consider not doing that. This
will be faster.

Test Plan:
```
$ time node bin/sourcecred.js load tensorflow/tensorflow --plugin git

real	0m33.512s
user	0m35.196s
sys	0m12.489s
```

Also, `yarn test --full` passes.

wchargin-branch: git-deforestation

src/plugins/git/cloneAndLoadRepository.js

@@ -7,7 +7,8 @@ import {loadRepository} from "./loadRepository";
 import type {Repo} from "../../core/repo";
 
 /**
- * Load Git Repository data from a fresh clone of a GitHub repo.
+ * Load Git repository data from a fresh clone of a GitHub repo. Loads
+ * commits only.
  *
  * @param {Repo} repo
  *   the GitHub repository to be cloned
@@ -19,7 +20,7 @@ export default function cloneAndLoadRepository(repo: Repo): Repository {
   const tmpdir = tmp.dirSync({unsafeCleanup: true});
   const git = localGit(tmpdir.name);
   git(["clone", cloneUrl, ".", "--quiet"]);
-  const result = loadRepository(tmpdir.name, "HEAD");
+  const result = loadRepository(tmpdir.name, "HEAD", "COMMITS_ONLY");
   tmpdir.removeCallback();
   return result;
 }

src/plugins/git/loadRepository.js

@@ -21,11 +21,21 @@ import {localGit} from "./gitUtils";
  */
 export function loadRepository(
   repositoryPath: string,
-  rootRef: string
+  rootRef: string,
+  mode: "FULL" | "COMMITS_ONLY" = "FULL"
 ): Repository {
   const git = localGit(repositoryPath);
   const commits = findCommits(git, rootRef);
-  const trees = findTrees(git, new Set(commits.map((x) => x.treeHash)));
+  const trees = (() => {
+    switch (mode) {
+      case "FULL":
+        return findTrees(git, new Set(commits.map((x) => x.treeHash)));
+      case "COMMITS_ONLY":
+        return [];
+      default:
+        throw new Error((mode: empty));
+    }
+  })();
   return {commits: objectMap(commits), trees: objectMap(trees)};
 }
 

src/plugins/git/loadRepository.test.js

@@ -29,6 +29,16 @@ describe("loadRepository", () => {
     );
   });
 
+  it("respects commits-only mode", () => {
+    const repository = createExampleRepo(mkdtemp());
+    const full = loadRepository(repository.path, "HEAD");
+    const commitsOnly = loadRepository(repository.path, "HEAD", "COMMITS_ONLY");
+    expect(commitsOnly).toEqual({
+      commits: full.commits,
+      trees: {},
+    });
+  });
+
   it("processes an old commit", () => {
     const repository = createExampleRepo(mkdtemp());
     const whole = loadRepository(repository.path, "HEAD");