Attempted workaround for git worktree issue

If the directory found as the git dir contains a file called `gitdir`,
attempts to resolve it to find the actual git directory.

For issue #75 or my issue #964

lib-extra/src/main/java/com/diffplug/spotless/extra/GitRatchet.java

@@ -191,21 +191,37 @@ protected Repository repositoryFor(Project project) throws IOException {
 		File dotGitPath = new File(dir, dotGit);
 
 		if (dotGitPath.isDirectory()) {
+			// Handle case where directory is a worktree
+			File mainRepoRefFile = new File(dotGitPath, "gitdir");
+			if (mainRepoRefFile.isFile()) {
+				return tryReadGitMeta(mainRepoRefFile)
+						.map(path -> getDotGitDir(dir, path))
+						.orElse(null);
+			}
+
 			return dotGitPath;
 		} else if (dotGitPath.isFile()) {
-			try {
-				String relativePath = new String(Files.readAllBytes(dotGitPath.toPath()), StandardCharsets.UTF_8)
-						.split(":")[1].trim();
-				return getDotGitDir(dir, relativePath);
-			} catch (IOException e) {
-				System.err.println("failed to parse git meta: " + e.getMessage());
-				return null;
-			}
+			return tryReadGitMeta(dotGitPath)
+					.map(str -> {
+						String relativePath = str.split(":")[1].trim();
+						return getDotGitDir(dir, relativePath);
+					})
+					.orElse(null);
 		} else {
 			return null;
 		}
 	}
 
+	private static Optional<String> tryReadGitMeta(File file) {
+		try {
+			String content = new String(Files.readAllBytes(file.toPath()), StandardCharsets.UTF_8).trim();
+			return Optional.of(content);
+		} catch (IOException e) {
+			System.err.println("failed to parse git meta: " + e.getMessage());
+			return Optional.empty();
+		}
+	}
+
 	private static boolean isGitRoot(File dir) {
 		File dotGit = getDotGitDir(dir, Constants.DOT_GIT);
 		return dotGit != null && RepositoryCache.FileKey.isGitRepository(dotGit, FS.DETECTED);