fix: short-circuit to avoid rebasing (#8622)

- Do not try to rebase a pull request when it is zero commits behind. We can trust this number as before merging a repository the status of the pull request is mergeable and thus not in a conflict checking stage (where this would be updated).
- This resolves a issue where `git-replay` would rebase a pull request when this is not needed and causes to lose the signature of Git commits and commit IDs as shown in the pullrequest commits timeline.
- Resolves forgejo/forgejo#8619
- Add a simple integration test that simply checks that after merging a up-to-date pull request via the rebase style that the commit ID didn't change. This demonstrates that it didn't do needlessly rebasing.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8622
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

services/pull/merge_prepare.go

@@ -249,6 +249,11 @@ func rebaseTrackingOnToBase(ctx *mergeContext, mergeStyle repo_model.MergeStyle)
 	ctx.outbuf.Reset()
 	ctx.errbuf.Reset()
 
+	// If the pull request is zero commits behind, then no rebasing needs to be done.
+	if ctx.pr.CommitsBehind == 0 {
+		return nil
+	}
+
 	// Check git version for availability of git-replay. If it is available, we use
 	// it for performance and to preserve unknown commit headers like the
 	// "change-id" header used by Jujutsu and GitButler to track changes across

tests/integration/pull_merge_test.go

@@ -1135,3 +1135,32 @@ func TestPullDeleteBranchPerms(t *testing.T) {
 		user4Session.MakeRequest(t, req, http.StatusOK)
 	})
 }
+
+// Test that rebasing only happens when its necessary.
+func TestRebaseWhenNecessary(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, giteaURL *url.URL) {
+		session := loginUser(t, "user1")
+		testRepoFork(t, session, "user2", "repo1", "user1", "repo1")
+		testEditFile(t, session, "user1", "repo1", "master", "README.md", "Hello, World (Edited)\n")
+
+		resp := testPullCreate(t, session, "user1", "repo1", false, "master", "master", "This is a pull title")
+		pullLink := test.RedirectURL(resp)
+
+		resp = session.MakeRequest(t, NewRequest(t, "GET", test.RedirectURL(resp)+"/commits"), http.StatusOK)
+		htmlDoc := NewHTMLParser(t, resp.Body)
+		commitLinkBefore, ok := htmlDoc.Find("a.sha").Attr("href")
+		assert.True(t, ok)
+		commitBefore := commitLinkBefore[strings.LastIndexByte(commitLinkBefore, '/'):]
+
+		elem := strings.Split(pullLink, "/")
+		testPullMerge(t, session, elem[1], elem[2], elem[4], repo_model.MergeStyleRebase, false)
+
+		resp = session.MakeRequest(t, NewRequest(t, "GET", "/user2/repo1"), http.StatusOK)
+		htmlDoc = NewHTMLParser(t, resp.Body)
+		commitLinkAfter, ok := htmlDoc.Find(".latest-commit a.sha").Attr("href")
+		assert.True(t, ok)
+		commitAfter := commitLinkAfter[strings.LastIndexByte(commitLinkAfter, '/'):]
+
+		assert.Equal(t, commitBefore, commitAfter)
+	})
+}