Prevent dangling GetAttribute calls (#18754)

It appears possible that there could be a hang due to unread data from the
repo-attribute command pipes. This PR simply closes these during the defer.

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/gitdiff/gitdiff.go

@@ -1422,6 +1422,7 @@ func GetDiff(gitRepo *git.Repository, opts *DiffOptions, files ...string) (*Diff
 				}()
 			}
 			defer func() {
+				_ = checker.Close()
 				cancel()
 			}()
 		}