enhance: allow auto-closing PRs from PRs (#39393)

On GitHub, one can close PRs via `Fixes: #123` references which was not
possible on Gitea before, but now is. Verified fully that behaviour
matches GH and ensured no regressions for external trackers.

services/issue/commit.go

@@ -18,6 +18,7 @@ import (
 	issues_model "gitea.dev/models/issues"
 	access_model "gitea.dev/models/perm/access"
 	repo_model "gitea.dev/models/repo"
+	"gitea.dev/models/unit"
 	user_model "gitea.dev/models/user"
 	"gitea.dev/modules/container"
 	"gitea.dev/modules/git"
@@ -188,8 +189,8 @@ func UpdateIssuesCommit(ctx context.Context, doer *user_model.User, repo *repo_m
 				return err
 			}
 
-			// Only issues can be closed/reopened this way, and user needs the correct permissions
-			if refIssue.IsPull || !canclose {
+			// Only issues can be reopened this way, and user needs the correct permissions
+			if !canclose || (refIssue.IsPull && ref.Action == references.XRefActionReopens) {
 				continue
 			}
 
@@ -198,6 +199,11 @@ func UpdateIssuesCommit(ctx context.Context, doer *user_model.User, repo *repo_m
 				continue
 			}
 
+			// With an external tracker, pull requests are referenced as "!N"
+			if !ref.IsPull && refIssue.IsPull && refRepo.UnitEnabled(ctx, unit.TypeExternalTracker) {
+				continue
+			}
+
 			if !repo.CloseIssuesViaCommitInAnyBranch {
 				// If the issue was specified to be in a particular branch, don't allow commits in other branches to close it
 				if refIssue.Ref != "" {