Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

modules/git/log_name_status.go

@@ -106,7 +106,7 @@ func (g *LogNameStatusRepoParser) Next(treepath string, paths2ids map[string]int
 			case bufio.ErrBufferFull:
 				g.buffull = true
 			case io.EOF:
-				return nil, nil
+				return nil, nil //nolint:nilnil // return nil to signal EOF
 			default:
 				return nil, err
 			}
@@ -121,7 +121,7 @@ func (g *LogNameStatusRepoParser) Next(treepath string, paths2ids map[string]int
 			case bufio.ErrBufferFull:
 				g.buffull = true
 			case io.EOF:
-				return nil, nil
+				return nil, nil //nolint:nilnil // return nil to signal EOF
 			default:
 				return nil, err
 			}