Fix tag handling: preserve annotations and explicit fetch-tags

This PR fixes several issues with tag handling in the checkout action:

1. fetch-tags: true now works (fixes #1471)
   - Tags refspec is now included in getRefSpec() when fetchTags=true
   - Previously tags were only fetched during a separate fetch that was
     overwritten by the main fetch

2. Tag checkout preserves annotations (fixes #290)
   - Tags are fetched via refspec (+refs/tags/*:refs/tags/*) instead of
     --tags flag
   - This fetches the actual tag objects, preserving annotations

3. Tag checkout with fetch-tags: true no longer fails (fixes #1467)
   - When checking out a tag with fetchTags=true, only the wildcard
     refspec is used (specific tag refspec is redundant)

Changes:
- src/ref-helper.ts: getRefSpec() now accepts fetchTags parameter and
  prepends tags refspec when true
- src/git-command-manager.ts: fetch() simplified to always use --no-tags,
  tags are fetched explicitly via refspec
- src/git-source-provider.ts: passes fetchTags to getRefSpec()
- Added E2E test for fetch-tags option

Related #1471, #1467, #290

src/git-source-provider.ts

@@ -159,7 +159,6 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
     const fetchOptions: {
       filter?: string
       fetchDepth?: number
-      fetchTags?: boolean
       showProgress?: boolean
     } = {}
 
@@ -182,12 +181,35 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
       if (!(await refHelper.testRef(git, settings.ref, settings.commit))) {
         refSpec = refHelper.getRefSpec(settings.ref, settings.commit)
         await git.fetch(refSpec, fetchOptions)
+
+        // Verify the ref now matches. For branches, the targeted fetch above brings
+        // in the specific commit. For tags (fetched by ref), this will fail if
+        // the tag was moved after the workflow was triggered.
+        if (!(await refHelper.testRef(git, settings.ref, settings.commit))) {
+          throw new Error(
+            `The ref '${settings.ref}' does not point to the expected commit '${settings.commit}'. ` +
+              `The ref may have been updated after the workflow was triggered.`
+          )
+        }
       }
     } else {
       fetchOptions.fetchDepth = settings.fetchDepth
-      fetchOptions.fetchTags = settings.fetchTags
-      const refSpec = refHelper.getRefSpec(settings.ref, settings.commit)
+      const refSpec = refHelper.getRefSpec(
+        settings.ref,
+        settings.commit,
+        settings.fetchTags
+      )
       await git.fetch(refSpec, fetchOptions)
+
+      // For tags, verify the ref still points to the expected commit.
+      // Tags are fetched by ref (not commit), so if a tag was moved after the
+      // workflow was triggered, we would silently check out the wrong commit.
+      if (!(await refHelper.testRef(git, settings.ref, settings.commit))) {
+        throw new Error(
+          `The ref '${settings.ref}' does not point to the expected commit '${settings.commit}'. ` +
+            `The ref may have been updated after the workflow was triggered.`
+        )
+      }
     }
     core.endGroup()