Fix git.Blob.DataAsync(): close pipe since we return a NopCloser (#16899) (#16900)

* make sure headGitRepo is closed on err too
* refactor
* Fix git.Blob.DataAsync(): exec cancel since we already read all bytes (close pipe since we return a NopCloser)

Co-authored-by: zeripath <art27@cantab.net>

routers/web/repo/compare.go

@@ -623,7 +623,6 @@ func CompareDiff(ctx *context.Context) {
 			headGitRepo.Close()
 		}
 	}()
-
 	if ctx.Written() {
 		return
 	}