feat(manager/git-submodules): Support for special branch value .

lib/modules/manager/git-submodules/__fixtures__/.gitmodules.7

@@ -0,0 +1,4 @@
+[submodule "PowerShell-Docs"]
+	path = PowerShell-Docs
+	url = git@github.com:PowerShell/PowerShell-Docs
+	branch = .

lib/modules/manager/git-submodules/extract.spec.ts

@@ -299,5 +299,42 @@ describe('modules/manager/git-submodules/extract', () => {
         ],
       });
     });
+
+    it('fallback to current branch if special value is detected', async () => {
+      gitMock.branch.mockResolvedValueOnce({
+        all: ['staging', 'main'],
+        branches: {
+          staging: {
+            current: true,
+            name: 'staging',
+            commit: '9eeb873',
+            label: 'staging branch',
+            linkedWorkTree: false,
+          },
+          main: {
+            current: false,
+            name: 'main',
+            commit: 'e14c7e1',
+            label: 'main branch',
+            linkedWorkTree: false,
+          },
+        },
+        current: 'staging',
+        detached: false,
+      });
+
+      const res = await extractPackageFile('', '.gitmodules.7', {});
+      expect(res).toEqual({
+        datasource: 'git-refs',
+        deps: [
+          {
+            currentDigest: '4b825dc642cb6eb9a060e54bf8d69288fbee4904',
+            currentValue: 'staging',
+            depName: 'PowerShell-Docs',
+            packageName: 'https://github.com/PowerShell/PowerShell-Docs',
+          },
+        ],
+      });
+    });
   });
 });

lib/modules/manager/git-submodules/extract.ts

@@ -54,19 +54,24 @@ async function getDefaultBranch(subModuleUrl: string): Promise<string> {
 }
 
 async function getBranch(
+  git: SimpleGit,
   gitModulesPath: string,
   submoduleName: string,
   subModuleUrl: string
 ): Promise<string> {
-  return (
-    (await Git(simpleGitConfig()).raw([
+  const branchFromConfig = (
+    await Git(simpleGitConfig()).raw([
       'config',
       '--file',
       gitModulesPath,
       '--get',
       `submodule.${submoduleName}.branch`,
-    ])) || (await getDefaultBranch(subModuleUrl))
+    ])
   ).trim();
+
+  return branchFromConfig === '.'
+    ? (await git.branch(['--show-current'])).current.trim()
+    : branchFromConfig || (await getDefaultBranch(subModuleUrl)).trim();
 }
 
 async function getModules(
@@ -123,6 +128,7 @@ export default async function extractPackageFile(
       const subModuleUrl = await getUrl(git, gitModulesPath, name);
       const httpSubModuleUrl = getHttpUrl(subModuleUrl);
       const currentValue = await getBranch(
+        git,
         gitModulesPath,
         name,
         httpSubModuleUrl