refactor: git repo and relative path handling (#38522)

1. simplify "StorageRepo" code
2. simplify git.OpenRepository code
3. by the way, clean up some unused files in git test fixtures.

models/repo/repo_location.go

@@ -0,0 +1,56 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"strconv"
+	"strings"
+
+	"gitea.dev/modules/git/gitcmd"
+)
+
+func repoCodeGitRepoRelativePath(ownerName, repoName string) string {
+	return strings.ToLower(ownerName) + "/" + strings.ToLower(repoName) + ".git"
+}
+
+func repoWikiGitRepoRelativePath(ownerName, repoName string) string {
+	return strings.ToLower(ownerName) + "/" + strings.ToLower(repoName) + ".wiki.git"
+}
+
+// CodeRepoByName returns an unmanaged repository facade for the code repository of the given owner and repository name.
+// Usually it is used for migration fixes or repository adoption/creation/rename/transfer.
+func CodeRepoByName(ownerName, repoName string) gitcmd.RepositoryFacade {
+	return gitcmd.RepositoryUnmanaged(repoCodeGitRepoRelativePath(ownerName, repoName))
+}
+
+func WikiRepoByName(ownerName, repoName string) gitcmd.RepositoryFacade {
+	return gitcmd.RepositoryUnmanaged(repoWikiGitRepoRelativePath(ownerName, repoName))
+}
+
+func repoCodeGitRepoManagedID(repoID int64) string {
+	return "repo-" + strconv.FormatInt(repoID, 10)
+}
+
+func (repo *Repository) CodeStorageRepo() gitcmd.RepositoryFacade {
+	id := repoCodeGitRepoManagedID(repo.ID)
+	repoPath := repoCodeGitRepoRelativePath(repo.OwnerName, repo.Name)
+	return gitcmd.RepositoryManaged(id, repoPath)
+}
+
+func (repo *Repository) GitRepoLocation() string {
+	// TODO: use CodeGitRepo instead of this one
+	return repoCodeGitRepoRelativePath(repo.OwnerName, repo.Name)
+}
+
+func (repo *Repository) GitRepoManagedID() string {
+	// TODO: use CodeGitRepo instead of this one
+	return repoCodeGitRepoManagedID(repo.ID)
+}
+
+func (repo *Repository) WikiStorageRepo() gitcmd.RepositoryFacade {
+	// The wiki repository should have the same object format as the code repository. TODO: old comment, REALLY? Why?
+	id := "repo-wiki-" + strconv.FormatInt(repo.ID, 10)
+	repoPath := repoWikiGitRepoRelativePath(repo.OwnerName, repo.Name)
+	return gitcmd.RepositoryManaged(id, repoPath)
+}