Keep the ignored files during the upgrade process

When the user adds new files to the .gitignore file,
those files are deleted during the upgrade process
because it starts by creating a fresh .gitignore file
without user's modification. This commit adds a step
to move the excluded file at the repository level so
that they are kept ignored during the upgrade process.

react-native-git-upgrade/cliEntry.js

@@ -147,6 +147,28 @@ function configureGitEnv(tmpDir) {
   process.env.GIT_WORK_TREE = '.';
 }
 
+function copyCurrentGitIgnoreFile(tmpDir) {
+  /*
+   * The user may have added new files or directories in the .gitignore file.
+   * We need to keep those files ignored during the process, otherwise they
+   * will be deleted.
+   * See https://github.com/facebook/react-native/issues/12237
+   */
+  try {
+    const gitignorePath = path.resolve(process.cwd(), '.gitignore');
+    const repoExcludePath = path.resolve(tmpDir, process.env.GIT_DIR, 'info/exclude');
+    const content = fs.readFileSync(gitignorePath, 'utf8');
+    fs.appendFileSync(repoExcludePath, content);
+  } catch (err) {
+    if (err.code === 'ENOENT') {
+      log.info('No .gitignore file found, this step is a no-op');
+      return;
+    }
+
+    throw err;
+  }
+}
+
 function generateTemplates(generatorDir, appName, verbose) {
   try {
     const yeomanGeneratorEntryPoint = path.resolve(generatorDir, 'index.js');
@@ -277,9 +299,12 @@ async function run(requestedVersion, cliArgs) {
     log.info('Configure Git environment');
     configureGitEnv(tmpDir);
 
-    log.info('Init Git repository');
+    log.info('Init temporary Git repository');
     await exec('git init', verbose);
 
+    log.info('Save current .gitignore file');
+    await copyCurrentGitIgnoreFile(tmpDir);
+
     log.info('Add all files to commit');
     await exec('git add .', verbose);