Concatenate error in `checkIfPRContentChanged` (#28731)

- If there's a error with the Git command in `checkIfPRContentChanged`
the stderr wasn't concatendated to the error, which results in still not
knowing why an error happend.
- Adds concatenation for stderr to the returned error.
- Ref: https://codeberg.org/forgejo/forgejo/issues/2077

Co-authored-by: Gusted <postmaster@gusted.xyz>

services/pull/pull.go

@@ -4,6 +4,7 @@
 package pull
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io"
@@ -422,9 +423,11 @@ func checkIfPRContentChanged(ctx context.Context, pr *issues_model.PullRequest,
 		return false, fmt.Errorf("unable to open pipe for to run diff: %w", err)
 	}
 
+	stderr := new(bytes.Buffer)
 	if err := cmd.Run(&git.RunOpts{
 		Dir:    prCtx.tmpBasePath,
 		Stdout: stdoutWriter,
+		Stderr: stderr,
 		PipelineFunc: func(ctx context.Context, cancel context.CancelFunc) error {
 			_ = stdoutWriter.Close()
 			defer func() {
@@ -436,6 +439,7 @@ func checkIfPRContentChanged(ctx context.Context, pr *issues_model.PullRequest,
 		if err == util.ErrNotEmpty {
 			return true, nil
 		}
+		err = git.ConcatenateError(err, stderr.String())
 
 		log.Error("Unable to run diff on %s %s %s in tempRepo for PR[%d]%s/%s...%s/%s: Error: %v",
 			newCommitID, oldCommitID, base,