Fix gitlab prepare fails with a 404.

A GitLab URL not ending in .git requires a redirect which isomorphic-git doesn't follow, resulting in a 404.
This incorporates @ruloweb's fix in #4134.

.changeset/breezy-meals-lie.md

@@ -16,3 +16,5 @@ catalog:
     - type: url
       target: https://github.com/backstage/backstage/blob/other/plugins/scaffolder-backend/sample-templates/docs-template/template.yaml
 ```
+
+This also fixes a 404 in the prepare stage for GitLab URLs.

plugins/scaffolder-backend/src/scaffolder/stages/prepare/gitlab.test.ts

@@ -87,7 +87,7 @@ describe('GitLabPreparer', () => {
     await preparer.prepare(mockEntity, { logger: getVoidLogger() });
 
     expect(mockGitClient.clone).toHaveBeenCalledWith({
-      url: 'https://gitlab.com/benjdlambert/backstage-graphql-template',
+      url: 'https://gitlab.com/benjdlambert/backstage-graphql-template.git',
       dir: expect.any(String),
       ref: expect.any(String),
     });
@@ -112,7 +112,7 @@ describe('GitLabPreparer', () => {
     await preparer.prepare(mockEntity, { logger: getVoidLogger() });
 
     expect(mockGitClient.clone).toHaveBeenCalledWith({
-      url: 'https://gitlab.com/benjdlambert/backstage-graphql-template',
+      url: 'https://gitlab.com/benjdlambert/backstage-graphql-template.git',
       dir: expect.any(String),
       ref: expect.any(String),
     });

plugins/scaffolder-backend/src/scaffolder/stages/prepare/gitlab.ts

@@ -41,6 +41,7 @@ export class GitlabPreparer implements PreparerBase {
     const templateId = template.metadata.name;
 
     const parsedGitLocation = parseGitUrl(location);
+    parsedGitLocation.git_suffix = true;
     const repositoryCheckoutUrl = parsedGitLocation.toString('https');
     const ref = parsedGitLocation.ref;
     const tempDir = await fs.promises.mkdtemp(