techdocs: Replace nodegit with isomorphic-git

TechDocs needs 4 functions from the git library
1. To clone 2. To get the commit timestamp for HEAD 3. To fetch 4. To merge two branches

Created necessary commands in the new SCM client and reordered them alphabetically for convenience

packages/techdocs-common/src/helpers.ts

@@ -17,19 +17,14 @@
 import os from 'os';
 import path from 'path';
 import parseGitUrl from 'git-url-parse';
-import NodeGit, { Clone, Repository } from 'nodegit';
 import fs from 'fs-extra';
 import { getDefaultBranch } from './default-branch';
 import { getGitRepoType, getTokenForGitRepo } from './git-auth';
 import { Entity } from '@backstage/catalog-model';
-import { InputError, UrlReader } from '@backstage/backend-common';
+import { InputError, UrlReader, Git } from '@backstage/backend-common';
 import { RemoteProtocol } from './stages/prepare/types';
 import { Logger } from 'winston';
 
-// Enables core.longpaths on windows to prevent crashing when checking out repos with long foldernames and/or deep nesting
-// @ts-ignore
-NodeGit.Libgit2.opts(28, 1);
-
 export type ParsedLocationAnnotation = {
   type: RemoteProtocol;
   target: string;
@@ -124,17 +119,56 @@ export const checkoutGitRepository = async (
   const repositoryTmpPath = await getGitRepositoryTempFolder(repoUrl);
   const token = await getTokenForGitRepo(repoUrl);
 
+  // Initialize a git client
+  let git = Git.fromAuth({ logger });
+
+  // Docs about why username and password are set to these specific values.
+  // https://isomorphic-git.org/docs/en/onAuth#oauth2-tokens
+  if (token) {
+    const type = getGitRepoType(repoUrl);
+    switch (type) {
+      case 'github':
+        git = Git.fromAuth({
+          username: token,
+          password: 'x-oauth-basic',
+          logger,
+        });
+        parsedGitLocation.token = `${token}:x-oauth-basic`;
+        break;
+      case 'gitlab':
+        git = Git.fromAuth({
+          username: 'oauth2',
+          password: token,
+          logger,
+        });
+        parsedGitLocation.token = `dummyUsername:${token}`;
+        parsedGitLocation.git_suffix = true;
+        break;
+      case 'azure/api':
+        git = Git.fromAuth({
+          username: 'notempty',
+          password: token,
+          logger: logger,
+        });
+        break;
+      default:
+        parsedGitLocation.token = `:${token}`;
+    }
+  }
+
+  // Pull from repository if it has already been cloned.
   if (fs.existsSync(repositoryTmpPath)) {
     try {
-      const repository = await Repository.open(repositoryTmpPath);
-      const currentBranchName = (
-        await repository.getCurrentBranch()
-      ).shorthand();
-      await repository.fetch('origin');
-      await repository.mergeBranches(
-        currentBranchName,
-        `origin/${currentBranchName}`,
-      );
+      const currentBranchName = await git.currentBranch({
+        dir: repositoryTmpPath,
+      });
+
+      await git.fetch({ dir: repositoryTmpPath, remote: 'origin' });
+      await git.merge({
+        dir: repositoryTmpPath,
+        headBranch: `origin/${currentBranchName}`,
+        baseBranch: currentBranchName || undefined,
+      });
       return repositoryTmpPath;
     } catch (e) {
       logger.info(
@@ -144,26 +178,10 @@ export const checkoutGitRepository = async (
     }
   }
 
-  if (token) {
-    const type = getGitRepoType(repoUrl);
-    switch (type) {
-      case 'gitlab':
-        // Personal Access Token
-        parsedGitLocation.token = `dummyUsername:${token}`;
-        parsedGitLocation.git_suffix = true;
-        break;
-      case 'github':
-        parsedGitLocation.token = `${token}:x-oauth-basic`;
-        break;
-      default:
-        parsedGitLocation.token = `:${token}`;
-    }
-  }
-
   const repositoryCheckoutUrl = parsedGitLocation.toString('https');
 
   fs.mkdirSync(repositoryTmpPath, { recursive: true });
-  await Clone.clone(repositoryCheckoutUrl, repositoryTmpPath);
+  await git.clone({ url: repositoryCheckoutUrl, dir: repositoryTmpPath });
 
   return repositoryTmpPath;
 };
@@ -174,10 +192,11 @@ export const getLastCommitTimestamp = async (
 ): Promise<number> => {
   const repositoryLocation = await checkoutGitRepository(repositoryUrl, logger);
 
-  const repository = await Repository.open(repositoryLocation);
-  const commit = await repository.getReferenceCommit('HEAD');
+  const git = Git.fromAuth({ logger });
+  const sha = await git.resolveRef({ dir: repositoryLocation, ref: 'HEAD' });
+  const commit = await git.readCommit({ dir: repositoryLocation, sha });
 
-  return commit.date().getTime();
+  return commit.commit.committer.timestamp;
 };
 
 export const getDocFilesFromRepository = async (