ci: Simplify run-eslint script

Now that all code conforms with the new style, we can remove all
the tricky bits that compare errors from regular- and legacy
config or limit checks to changed lines from a git diff.

All that is left over the eslint CLI tool is the ability to output
results both as junit for gitlab and plain text for logs without
duplicating the linting, but that's well worth preserving.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2866>

.gitignore

@@ -4,6 +4,5 @@ po/gnome-shell.pot
 *.patch
 *.sw?
 .buildconfig
-.eslintcache-legacy
+.eslintcache
-.eslintcache-regular
 .vscode

.gitlab-ci/run-eslint

@@ -6,80 +6,6 @@ console.log(`Running ESLint version ${ESLint.version}...`);
 
 const fs = require('fs');
 const path = require('path');
-const { spawn } = require('child_process');
-
-function createConfig(config) {
-    const options = {
-        cache: true,
-        cacheLocation: `.eslintcache-${config}`,
-    };
-
-    if (config === 'legacy')
-        options.overrideConfigFile='lint/eslintrc-legacy.yml';
-
-    return new ESLint(options);
-}
-
-function git(...args) {
-    const git = spawn('git', args, { stdio: ['ignore', null, 'ignore'] });
-    git.stdout.setEncoding('utf8');
-
-    return new Promise(resolve => {
-        let out = '';
-        git.stdout.on('data', chunk => out += chunk);
-        git.stdout.on('end', () => resolve(out.trim()));
-    });
-}
-
-function createCommon(report1, report2, ignoreColumn=false) {
-    return report1.map(result => {
-        const { filePath, messages } = result;
-        const match =
-            report2.find(r => r.filePath === filePath) || { messages: [] };
-
-        const filteredMessages = messages.filter(
-            msg => match.messages.some(
-                m => m.line === msg.line && (ignoreColumn || m.column === msg.column)));
-
-        const [errorCount, warningCount] = filteredMessages.reduce(
-            ([e, w], msg) => {
-                return [
-                    e + Number(msg.severity === 2),
-                    w + Number(msg.severity === 1)];
-            }, [0, 0]);
-
-        return {
-            filePath,
-            messages: filteredMessages,
-            errorCount,
-            warningCount,
-        };
-    });
-}
-
-async function getMergeRequestChanges(remote, branch) {
-    await git('fetch', remote, branch);
-    const branchPoint = await git('merge-base', 'HEAD', 'FETCH_HEAD');
-    const diff = await git('diff', '-U0', `${branchPoint}...HEAD`);
-
-    const report = [];
-    let messages = null;
-    for (const line of diff.split('\n')) {
-        if (line.startsWith('+++ b/')) {
-            const filePath = path.resolve(line.substring(6));
-            messages = filePath.endsWith('.js') ? [] : null;
-            if (messages)
-                report.push({ filePath, messages });
-        } else if (messages && line.startsWith('@@ ')) {
-            [, , changes] = line.split(' ');
-            [start, count] = `${changes},1`.split(',').map(i => parseInt(i));
-            for (let i = start; i < start + count; i++)
-                messages.push({ line: i });
-        }
-    }
-
-    return report;
-}
 
 function hasOption(...names) {
     return process.argv.some(arg => names.includes(arg));
@@ -102,38 +28,26 @@ function getOption(...names) {
     const sourceDir = path.dirname(process.argv[1]);
     process.chdir(path.resolve(sourceDir, '..'));
 
-    const remote = getOption('--remote') || 'origin';
-    const branch = getOption('--branch', '-b');
-
     const sources = ['js', 'subprojects/extensions-app/js', 'tests'];
-    const regular = createConfig('regular');
+    const eslint = new ESLint({cache: true});
 
-    const ops = [];
+    const results = await eslint.lintFiles(sources);
-    ops.push(regular.lintFiles(sources));
+    const formatter = await eslint.loadFormatter(getOption('--format', '-f'));
-    if (branch)
+    const resultText = formatter.format(results);
-        ops.push(getMergeRequestChanges(remote, branch));
-    else
-        ops.push(createConfig('legacy').lintFiles(sources));
-
-    const results = await Promise.all(ops);
-    const commonResults = createCommon(...results, branch !== undefined);
-
-    const formatter = await regular.loadFormatter(getOption('--format', '-f'));
-    const resultText = formatter.format(commonResults);
 
     if (outputPath) {
         fs.mkdirSync(path.dirname(outputPath), { recursive: true });
         fs.writeFileSync(outputPath, resultText);
 
         if (hasOption('--stdout')) {
-            const consoleFormatter = await regular.loadFormatter();
+            const consoleFormatter = await eslint.loadFormatter();
-            console.log(consoleFormatter.format(commonResults));
+            console.log(consoleFormatter.format(results));
         }
     } else {
         console.log(resultText);
     }
 
-    process.exitCode = commonResults.some(r => r.errorCount > 0) ? 1 : 0;
+    process.exitCode = results.some(r => r.errorCount > 0) ? 1 : 0;
 })().catch((error) => {
     process.exitCode = 1;
     console.error(error);