Fix #646: add tools/get-filer-version.js to find old Filer versions

tests/filesystems/images/README.md

@@ -5,3 +5,5 @@ generate them:
 
 1. `tiny-fs.0.43.json` was created from `tests/filesystem/tiny-fs/` with https://github.com/filerjs/filer/blob/d66114e20c7f0235698d9933dbd90217ba86fa4e/dist/filer.min.js
 
+If you need to create a new image, use `tools/get-filer-version.js` to
+get a specific version of Filer, then `tools/fs-image.js` to generate an image.

tools/get-filer-version.js

@@ -0,0 +1,39 @@
+#!/usr/bin/env node
+/* eslint-disable no-console */
+'use strict';
+
+const {spawn} = require('child_process');
+const meow = require('meow');
+
+const cli = meow(`
+	Usage
+    $ get-filer-version <SHA|branch|tag> [--out path/to/filer.js]
+
+  Options
+    --out, -o    Specify a Filer module path to use for output
+
+  Examples
+    $ get-filer-version v0.0.44
+    $ get-filer-version v0.0.44 --out filer.js
+`, {
+  description: 'Try to get an old version of Filer based on git SHA, branch, or tag',
+  flags: {
+    out: {
+      type: 'string',
+      alias: 'o'
+    }
+  }
+});
+
+// Get arg list, make sure we get a SHA argument
+cli.flags.app = cli.input.slice(1);
+if(!(cli.input && cli.input.length === 1)) {
+  console.error('Specify a git SHA, branch or tag to use');
+  process.exit(1);
+}
+
+const sha = cli.input[0];
+const out = cli.flags.out || `filer-${sha}.js`;
+// https://stackoverflow.com/questions/888414/git-checkout-older-revision-of-a-file-under-a-new-name?answertab=active#tab-top
+const cmd = `git show ${sha}:dist/filer.js > ${out}`;
+spawn (cmd, [], {stdio: 'inherit', shell: true});