Refactor git command stderr handling (#36402)

And clean up legacy fragile & incorrect logic

modules/git/gitcmd/utils.go

@@ -4,22 +4,9 @@
 package gitcmd
 
 import (
-	"fmt"
 	"io"
-
-	"code.gitea.io/gitea/modules/util"
 )
 
-// ConcatenateError concatenates an error with stderr string
-// FIXME: use RunStdError instead
-func ConcatenateError(err error, stderr string) error {
-	if len(stderr) == 0 {
-		return err
-	}
-	errMsg := fmt.Sprintf("%s - %s", err.Error(), stderr)
-	return util.ErrorWrap(&runStdError{err: err, stderr: stderr, errMsg: errMsg}, "%s", errMsg)
-}
-
 func safeClosePtrCloser[T *io.ReadCloser | *io.WriteCloser](c T) {
 	switch v := any(c).(type) {
 	case *io.ReadCloser: