Recognize git worktree directories as valid git repositories

spec/fixtures/git/master.git/config

@@ -7,3 +7,5 @@
 [remote "origin"]
 	url = https://github.com/example-user/example-repo.git
 	fetch = +refs/heads/*:refs/remotes/origin/*
+[gc]
+	worktreePruneExpire = never

spec/fixtures/git/master.git/worktrees/worktree-dir/HEAD

@@ -0,0 +1 @@
+ef046e9eecaa5255ea5e9817132d4001724d6ae1

spec/fixtures/git/master.git/worktrees/worktree-dir/commondir

@@ -0,0 +1 @@
+../..

spec/git-repository-provider-spec.js

@@ -97,6 +97,25 @@ describe('GitRepositoryProvider', () => {
       });
     });
 
+    describe('when specified a Directory with a commondir file for a worktree', () => {
+      it('returns a Promise that resolves to a GitRepository', async () => {
+        const directory = new Directory(
+          path.join(
+            __dirname,
+            'fixtures',
+            'git',
+            'master.git',
+            'worktrees',
+            'worktree-dir'
+          )
+        );
+        const result = await provider.repositoryForDirectory(directory);
+        expect(result).toBeInstanceOf(GitRepository);
+        expect(provider.pathToRepository[result.getPath()]).toBeTruthy();
+        expect(result.getType()).toBe('git');
+      });
+    });
+
     describe('when specified a Directory without exists()', () => {
       let directory;
 
@@ -193,6 +212,25 @@ describe('GitRepositoryProvider', () => {
       });
     });
 
+    describe('when specified a Directory with a commondir file for a worktree', () => {
+      it('returns a Promise that resolves to a GitRepository', () => {
+        const directory = new Directory(
+          path.join(
+            __dirname,
+            'fixtures',
+            'git',
+            'master.git',
+            'worktrees',
+            'worktree-dir'
+          )
+        );
+        const result = provider.repositoryForDirectorySync(directory);
+        expect(result).toBeInstanceOf(GitRepository);
+        expect(provider.pathToRepository[result.getPath()]).toBeTruthy();
+        expect(result.getType()).toBe('git');
+      });
+    });
+
     describe('when specified a Directory without existsSync()', () => {
       let directory;
 

src/git-repository-provider.js

@@ -99,10 +99,22 @@ function isValidGitDirectorySync(directory) {
   // To decide whether a directory has a valid .git folder, we use
   // the heuristic adopted by the valid_repository_path() function defined in
   // node_modules/git-utils/deps/libgit2/src/repository.c.
+  const commonDirFile = directory.getSubdirectory('commondir');
+  let commonDir;
+  if (commonDirFile.existsSync()) {
+    const commonDirPathBuff = fs.readFileSync(commonDirFile.getPath());
+    const commonDirPathString = commonDirPathBuff.toString().trim();
+    commonDir = new Directory(directory.resolve(commonDirPathString));
+    if (!commonDir.existsSync()) {
+      return false;
+    }
+  } else {
+    commonDir = directory;
+  }
   return (
-    directory.getSubdirectory('objects').existsSync() &&
     directory.getFile('HEAD').existsSync() &&
-    directory.getSubdirectory('refs').existsSync()
+    commonDir.getSubdirectory('objects').existsSync() &&
+    commonDir.getSubdirectory('refs').existsSync()
   );
 }
 
@@ -114,10 +126,22 @@ async function isValidGitDirectory(directory) {
   // To decide whether a directory has a valid .git folder, we use
   // the heuristic adopted by the valid_repository_path() function defined in
   // node_modules/git-utils/deps/libgit2/src/repository.c.
+  const commonDirFile = directory.getSubdirectory('commondir');
+  let commonDir;
+  if (await commonDirFile.exists()) {
+    const commonDirPathBuff = await fs.readFile(commonDirFile.getPath());
+    const commonDirPathString = commonDirPathBuff.toString().trim();
+    commonDir = new Directory(directory.resolve(commonDirPathString));
+    if (!(await commonDir.exists())) {
+      return false;
+    }
+  } else {
+    commonDir = directory;
+  }
   return (
-    (await directory.getSubdirectory('objects').exists()) &&
     (await directory.getFile('HEAD').exists()) &&
-    directory.getSubdirectory('refs').exists()
+    (await commonDir.getSubdirectory('objects').exists()) &&
+    commonDir.getSubdirectory('refs').exists()
   );
 }