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

* 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)

routers/web/repo/pull.go

@@ -1024,10 +1024,14 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 	)
 
 	headUser, headRepo, headGitRepo, prInfo, baseBranch, headBranch := ParseCompareInfo(ctx)
+	defer func() {
+		if headGitRepo != nil {
+			headGitRepo.Close()
+		}
+	}()
 	if ctx.Written() {
 		return
 	}
-	defer headGitRepo.Close()
 
 	labelIDs, assigneeIDs, milestoneID, _ := ValidateRepoMetas(ctx, *form, true)
 	if ctx.Written() {