fix pulls broken when fork repository deleted (#6754)

* fix pulls broken when fork repository deleted

* fix lint

models/error.go

@@ -1091,6 +1091,24 @@ func (err ErrPullRequestAlreadyExists) Error() string {
 		err.ID, err.IssueID, err.HeadRepoID, err.BaseRepoID, err.HeadBranch, err.BaseBranch)
 }
 
+// ErrPullRequestHeadRepoMissing represents a "ErrPullRequestHeadRepoMissing" error
+type ErrPullRequestHeadRepoMissing struct {
+	ID         int64
+	HeadRepoID int64
+}
+
+// IsErrErrPullRequestHeadRepoMissing checks if an error is a ErrPullRequestHeadRepoMissing.
+func IsErrErrPullRequestHeadRepoMissing(err error) bool {
+	_, ok := err.(ErrPullRequestHeadRepoMissing)
+	return ok
+}
+
+// Error does pretty-printing :D
+func (err ErrPullRequestHeadRepoMissing) Error() string {
+	return fmt.Sprintf("pull request head repo missing [id: %d, head_repo_id: %d]",
+		err.ID, err.HeadRepoID)
+}
+
 // ErrInvalidMergeStyle represents an error if merging with disabled merge strategy
 type ErrInvalidMergeStyle struct {
 	ID    int64