fix(repo): commit page fails to render unsigned commits with a different committer (#39381)

Since #39229 the commit page header dereferences
`.Verification.CommittingUser` when the committer is not the author.
`Verification` is `nil` for unsigned commits (see `repo.Diff`), so
opening such a commit — a rebased or cherry-picked one, for example —
logs a template error and the page comes out truncated:

```
Render failed: failed to render template: repo/commit_page, error: template error: builtin(bindata):repo/commit_page:138:22 : executing "repo/commit_page" at <.Verification.CommittingUser>: nil pointer evaluating interface {}.CommittingUser
```

This guards the access and adds an integration test that creates a
commit with distinct author and committer identities and checks the page
renders completely (the status stays 200 on a mid-render failure, so the
test looks at the body).

_The fix was worked out with help from an AI assistant; I reviewed and
tested it myself._

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

tests/integration/integration_test.go

@@ -31,6 +31,7 @@ import (
 	"gitea.dev/modules/web"
 	"gitea.dev/modules/web/middleware"
 	"gitea.dev/routers"
+	"gitea.dev/routers/common"
 	gitea_context "gitea.dev/services/context"
 	"gitea.dev/tests"
 
@@ -289,6 +290,9 @@ func MakeRequest(t testing.TB, rw *RequestWrapper, expectedStatus int) *httptest
 			// don't use "require" which exits the test case and makes "wait group" wait forever
 			assert.Equal(t, expectedStatus, recorder.Code, "Request: %s %s", req.Method, req.URL.String())
 		}
+		if expectedStatus != http.StatusInternalServerError {
+			assert.NotContains(t, recorder.Body.String(), common.PageInternalServerErrorMark, "Request: %s %s, response should not contain internal server error", req.Method, req.URL.String())
+		}
 	}
 	return recorder
 }

tests/integration/repo_commits_test.go

@@ -14,7 +14,10 @@ import (
 	auth_model "gitea.dev/models/auth"
 	"gitea.dev/models/db"
 	git_model "gitea.dev/models/git"
+	repo_model "gitea.dev/models/repo"
+	"gitea.dev/models/unittest"
 	"gitea.dev/modules/commitstatus"
+	"gitea.dev/modules/git"
 	"gitea.dev/modules/json"
 	"gitea.dev/modules/setting"
 	api "gitea.dev/modules/structs"
@@ -77,7 +80,6 @@ func TestRepoCommits(t *testing.T) {
 		const (
 			commitID              = "5099b81332712fe655e34e8dd63574f503f61811"
 			expectedCommitterTime = "2017-08-06T19:56:13+02:00"
-			authorTime            = "2017-08-06T19:55:01+02:00"
 		)
 
 		req := NewRequest(t, "GET", "/user2/repo16/commits/branch/master")
@@ -104,6 +106,24 @@ func TestRepoCommits(t *testing.T) {
 		authorElem := doc.doc.Find(".latest-commit .avatar-stack-names")
 		assert.Equal(t, "6543", strings.TrimSpace(authorElem.Text()))
 	})
+
+	t.Run("CommitterIsNotAuthor", func(t *testing.T) {
+		repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+		err := git.ForceFastImport(t.Context(), repo1, []git.FastImportCommit{
+			{
+				Ref:       "refs/heads/test-branch-committer",
+				Files:     []git.FastImportFile{{Path: "dummy-file.txt", Content: "dummy-content"}},
+				Author:    &git.Signature{Name: "real-commit-author", Email: "dummy-email1@example.com"},
+				Committer: &git.Signature{Name: "non-author-committer", Email: "dummy-email2@example.com"},
+			},
+		})
+		require.NoError(t, err)
+		commitID, err := git.GetBranchCommitID(t.Context(), repo1, "test-branch-committer")
+		require.NoError(t, err)
+		req := NewRequest(t, "GET", "/user2/repo1/commit/"+commitID)
+		resp := session.MakeRequest(t, req, http.StatusOK)
+		assert.Contains(t, resp.Body.String(), "non-author-committer")
+	})
 }
 
 func TestRepoCommitsWithStatus(t *testing.T) {