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>

modules/git/fastimport.go

@@ -29,6 +29,8 @@ type FastImportCommit struct {
 	Ref     string
 	Message string
 	Files   []FastImportFile
+
+	Author, Committer *Signature
 }
 
 // ForceFastImportWithInit is for mainly for testing purpose
@@ -48,15 +50,32 @@ func ForceFastImportWithInit(ctx context.Context, repoLocalPath string, commits
 
 // ForceFastImport is for mainly for testing purpose
 func ForceFastImport(ctx context.Context, repo RepositoryFacade, commits []FastImportCommit) error {
-	var buf bytes.Buffer
+	buf := &bytes.Buffer{}
 	for i, c := range commits {
-		msg := util.IfZero(c.Message, fmt.Sprintf("commit %d", i+1))
-		_, _ = fmt.Fprintf(&buf, "reset %s\n", c.Ref)
-		_, _ = fmt.Fprintf(&buf, "commit %s\nmark :%d\ncommitter Gitea <gitea@example.com> 1500000000 +0000\n", c.Ref, i+1)
-		_, _ = fmt.Fprintf(&buf, "data %d\n%s\n", len(msg), msg)
+		_, _ = fmt.Fprintf(buf, "reset %s\n", c.Ref)
+		_, _ = fmt.Fprintf(buf, "commit %s\n", c.Ref)
+		_, _ = fmt.Fprintf(buf, "mark :%d\n", i+1)
+
+		if c.Author != nil {
+			buf.WriteString("author ")
+			_ = c.Author.Encode(buf)
+			buf.WriteByte('\n')
+		}
+		if c.Committer != nil {
+			buf.WriteString("committer ")
+			_ = c.Committer.Encode(buf)
+			buf.WriteByte('\n')
+		} else {
+			// "committer" is required, so we use a default one if not provided
+			buf.WriteString("committer Gitea <gitea@example.com> 1500000000 +0000\n")
+		}
+
+		msg := util.IfZero(c.Message, fmt.Sprintf("test commit %d", i+1))
+		_, _ = fmt.Fprintf(buf, "data %d\n%s\n", len(msg), msg)
+
 		for _, f := range c.Files {
 			mode := util.IfZero(f.Mode, EntryModeBlob)
-			_, _ = fmt.Fprintf(&buf, "M %s inline %s\ndata %d\n%s\n", mode.String(), f.Path, len(f.Content), f.Content)
+			_, _ = fmt.Fprintf(buf, "M %s inline %s\ndata %d\n%s\n", mode.String(), f.Path, len(f.Content), f.Content)
 		}
 	}
 	buf.WriteString("done\n")

modules/git/signature_nogogit.go

@@ -8,6 +8,7 @@ package git
 
 import (
 	"fmt"
+	"io"
 	"time"
 
 	"gitea.dev/modules/util"
@@ -24,7 +25,13 @@ func (s *Signature) String() string {
 	return fmt.Sprintf("%s <%s>", s.Name, s.Email)
 }
 
-// Decode decodes a byte array representing a signature to signature
+// Encode writes the signature for git commit object (same as gogit's object.Signature Encode method)
+func (s *Signature) Encode(w io.Writer) error {
+	_, err := fmt.Fprintf(w, "%s <%s> %d %s", s.Name, s.Email, max(0, s.When.Unix()), s.When.Format("-0700"))
+	return err
+}
+
+// Decode parses the signature for git commit object (same as gogit's object.Signature Decode method)
 func (s *Signature) Decode(b []byte) {
 	*s = *parseSignatureFromCommitLine(util.UnsafeBytesToString(b))
 }