refactor(gitfs): throw error if branch no longer exists

lib/platform/git/storage.js

@@ -227,8 +227,8 @@ class Storage {
       if (branchName) {
         const exists = await branchExists(branchName);
         if (!exists) {
-          logger.warn({ branchName }, 'getFile branch does not exist');
-          return null;
+          logger.info({ branchName }, 'branch no longer exists - aborting');
+          throw new Error('repository-changed');
         }
       }
       try {

test/platform/git/__snapshots__/storage.spec.js.snap

@@ -7,6 +7,8 @@ Array [
 ]
 `;
 
+exports[`platform/git/storage getFile(filePath, branchName) returns null for 404 1`] = `[Error: repository-changed]`;
+
 exports[`platform/git/storage getFileList() should return the correct files 1`] = `
 Array [
   "future_file",

test/platform/git/storage.spec.js

@@ -167,8 +167,13 @@ describe('platform/git/storage', () => {
       expect(res).toBe(null);
     });
     it('returns null for 404', async () => {
-      const res = await git.getFile('some-path', 'some-branch');
-      expect(res).toBe(null);
+      let e;
+      try {
+        await git.getFile('some-path', 'some-branch');
+      } catch (err) {
+        e = err;
+      }
+      expect(e).toMatchSnapshot();
     });
   });
   describe('commitFilesToBranch(branchName, files, message, parentBranch)', () => {