guessReleaseNotes(msys2-runtime): link to Cygwin's release notes

When a `component-update` ticket notices that a new Cygwin runtime
version is available, the plan is to allow calling the usual `/open pr`
and `/add release note` dance.

The `/open pr` support will take a bit more work, as a merging rebase is
involved that requires a PR at
https://github.com/git-for-windows/msys2-runtime, and once merged, a
follow-up one at https://github.com/git-for-windows/MSYS2-packages.

But we can already tackle (and use) the `/add release note` part. This
here patch forward-ports how things were done before GitHub Actions:
https://github.com/git-for-windows/build-extra/blob/356779ddb189/please.sh#L1727-L1730

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

GitForWindowsHelper/component-updates.js

@@ -55,11 +55,31 @@ const needsSeparateARM64Build = package_name => {
     ].includes(package_name)
 }
 
+const guessCygwinReleaseNotesURL = async (version) => {
+    const { fetchHTML } = require('./https-request')
+    const html = await fetchHTML('https://cygwin.com')
+    const match = html.match(new RegExp(`The most recent version of the Cygwin DLL is[^]*?<a href=['"]?([^"' ]*)[^>]*>${version}</a>`))
+    if (match) return match[1]
+
+    throw new Error(`Could not determine Cygwin Release Notes URL for version ${version}`)
+}
+
 const guessReleaseNotes = async (context, issue) => {
     if (!issue.pull_request
         &&issue.labels.filter(label => label.name === 'component-update').length !== 1) throw new Error(`Cannot determine release note from issue ${issue.number}`)
     let { package_name, version } = guessComponentUpdateDetails(issue.title, issue.body)
 
+    if (package_name === 'msys2-runtime') {
+        const url = await guessCygwinReleaseNotesURL(version)
+        const message = `Comes with the MSYS2 runtime (Git for Windows flavor) based on [Cygwin v${version}](${url}).`
+        return {
+            type: 'feature',
+            message,
+            package: package_name,
+            version
+        }
+    }
+
     const matchURLInIssue = (issue) => {
         const pattern = {
             bash: /(?:^|\n)(https:\/\/\S+)/, // use the first URL

__tests__/component-updates.test.js

@@ -56,6 +56,21 @@ jest.mock('../GitForWindowsHelper/github-api-request', () => {
     return mockGithubApiRequest
 })
 
+const mockFetchHTML = {
+    'https://cygwin.com': `<div>
+
+<h2 class="cartouche">Cygwin version</h2>
+
+<p>
+    The most recent version of the Cygwin DLL is
+    <b><a href="https://cygwin.com/pipermail/cygwin-announce/2023-September/011291.html">3.4.9</a></b>.
+</p>
+<p>
+    The Cygwin DLL currently works with all recent, commercially released
+    x86_64 versions of Windows, starting with Windows 7.
+</p>
+</div>`
+}
 const missingURL = 'https://wingit.blob.core.windows.net/x86-64/curl-8.1.2-1-x86_64.pkg.tar.xz'
 const missingMinTTYURL = 'https://wingit.blob.core.windows.net/i686/mintty-1~3.6.5-1-i686.pkg.tar.xz'
 const bogus32BitMSYS2RuntimeURL = 'https://wingit.blob.core.windows.net/i686/msys2-runtime-3.4.9-1-i686.pkg.tar.xz'
@@ -64,7 +79,10 @@ const mockDoesURLReturn404 = jest.fn(url => [
     missingURL, missingMinTTYURL, bogus32BitMSYS2RuntimeURL, bogus64BitMSYS2RuntimeURL
 ].includes(url))
 jest.mock('../GitForWindowsHelper/https-request', () => {
-    return { doesURLReturn404: mockDoesURLReturn404, }
+    return {
+        doesURLReturn404: mockDoesURLReturn404,
+        fetchHTML: jest.fn(url => mockFetchHTML[url])
+    }
 })
 
 
@@ -139,6 +157,17 @@ http://www.gnutls.org/news.html#2023-02-10`
         package: 'openssl',
         version: '3.1.1'
     })
+
+    expect(await guessReleaseNotes(context, {
+        labels: [{ name: 'component-update' }],
+        title: '[New cygwin version] cygwin-3.4.9',
+        body: `\nCygwin 3.4.9 release\n\nhttps://github.com/cygwin/cygwin/releases/tag/cygwin-3.4.9`
+    })).toEqual({
+        type: 'feature',
+        message: 'Comes with the MSYS2 runtime (Git for Windows flavor) based on [Cygwin v3.4.9](https://cygwin.com/pipermail/cygwin-announce/2023-September/011291.html).',
+        package: 'msys2-runtime',
+        version: '3.4.9'
+    })
 })
 
 test('getMissingDeployments()', async () => {