Concatenate error in `checkIfPRContentChanged` (#28731) (#28737)

Concatenate error in `checkIfPRContentChanged` (#28731) (#28737)

Backport #28731 by @earl-warren

- 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: Earl Warren <109468362+earl-warren@users.noreply.github.com>
Co-authored-by: Gusted <postmaster@gusted.xyz>