feat(dev-infra): Add support for formatting all staged files (angular#38402)

Adds an ng-dev formatter option to format all of the staged files. This will can
be used to format only the staged files during the pre-commit hook.

PR Close angular#38402

Author: josephperrott

dev-infra/format/cli.ts

@@ -7,7 +7,7 @@
  */
 import * as yargs from 'yargs';
 
-import {allChangedFilesSince, allFiles} from '../utils/repo-files';
+import {allChangedFilesSince, allFiles, allStagedFiles} from '../utils/repo-files';
 
 import {checkFiles, formatFiles} from './format';
 
@@ -34,6 +34,12 @@ export function buildFormatParser(localYargs: yargs.Argv) {
             const executionCmd = check ? checkFiles : formatFiles;
             executionCmd(allChangedFilesSince(sha));
           })
+      .command(
+          'staged', 'Run the formatter on all staged files', {},
+          ({check}) => {
+            const executionCmd = check ? checkFiles : formatFiles;
+            executionCmd(allStagedFiles());
+          })
       .command('files <files..>', 'Run the formatter on provided files', {}, ({check, files}) => {
         const executionCmd = check ? checkFiles : formatFiles;
         executionCmd(files);

dev-infra/utils/repo-files.ts

@@ -27,6 +27,18 @@ export function allChangedFilesSince(sha = 'HEAD') {
   return Array.from(new Set([...diffFiles, ...untrackedFiles]));
 }
 
+/**
+ * A list of all staged files which have been modified.
+ *
+ * Only added, created and modified files are listed as others (deleted, renamed, etc) aren't
+ * changed or available as content to act upon.
+ */
+export function allStagedFiles() {
+  return gitOutputAsArray(`git diff --staged --name-only --diff-filter=ACM`);
+}
+
+
+
 export function allFiles() {
   return gitOutputAsArray(`git ls-files`);
 }