Add IsErrRepoFilesAlreadyExist check when fork repo (#24678)

Before:

![image](https://github.com/go-gitea/gitea/assets/18380374/e9e3de75-eb6e-418e-9227-ef4e5f448a04)
After:

![image](https://github.com/go-gitea/gitea/assets/18380374/74b1cd6c-c5b1-4ee0-a1d8-6b3fb8d3a8e9)

Author: yp05327

routers/api/v1/repo/fork.go

@@ -5,6 +5,7 @@
 package repo
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -15,6 +16,7 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/context"
 	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/routers/api/v1/utils"
 	"code.gitea.io/gitea/services/convert"
@@ -141,7 +143,7 @@ func CreateFork(ctx *context.APIContext) {
 		Description: repo.Description,
 	})
 	if err != nil {
-		if repo_service.IsErrForkAlreadyExist(err) || repo_model.IsErrRepoAlreadyExist(err) || repo_model.IsErrReachLimitOfRepo(err) {
+		if errors.Is(err, util.ErrAlreadyExist) || repo_model.IsErrReachLimitOfRepo(err) {
 			ctx.Error(http.StatusConflict, "ForkRepository", err)
 		} else {
 			ctx.Error(http.StatusInternalServerError, "ForkRepository", err)

routers/web/repo/pull.go

@@ -272,6 +272,17 @@ func ForkPost(ctx *context.Context) {
 			ctx.RenderWithErr(msg, tplFork, &form)
 		case repo_model.IsErrRepoAlreadyExist(err):
 			ctx.RenderWithErr(ctx.Tr("repo.settings.new_owner_has_same_repo"), tplFork, &form)
+		case repo_model.IsErrRepoFilesAlreadyExist(err):
+			switch {
+			case ctx.IsUserSiteAdmin() || (setting.Repository.AllowAdoptionOfUnadoptedRepositories && setting.Repository.AllowDeleteOfUnadoptedRepositories):
+				ctx.RenderWithErr(ctx.Tr("form.repository_files_already_exist.adopt_or_delete"), tplFork, form)
+			case setting.Repository.AllowAdoptionOfUnadoptedRepositories:
+				ctx.RenderWithErr(ctx.Tr("form.repository_files_already_exist.adopt"), tplFork, form)
+			case setting.Repository.AllowDeleteOfUnadoptedRepositories:
+				ctx.RenderWithErr(ctx.Tr("form.repository_files_already_exist.delete"), tplFork, form)
+			default:
+				ctx.RenderWithErr(ctx.Tr("form.repository_files_already_exist"), tplFork, form)
+			}
 		case db.IsErrNameReserved(err):
 			ctx.RenderWithErr(ctx.Tr("repo.form.name_reserved", err.(db.ErrNameReserved).Name), tplFork, &form)
 		case db.IsErrNamePatternNotAllowed(err):