Simplify how git repositories are opened (#28937)

## Purpose
This is a refactor toward building an abstraction over managing git
repositories.
Afterwards, it does not matter anymore if they are stored on the local
disk or somewhere remote.

## What this PR changes
We used `git.OpenRepository` everywhere previously.
Now, we should split them into two distinct functions:

Firstly, there are temporary repositories which do not change:

```go
git.OpenRepository(ctx, diskPath)
```

Gitea managed repositories having a record in the database in the
`repository` table are moved into the new package `gitrepo`:

```go
gitrepo.OpenRepository(ctx, repo_model.Repo)
```

Why is `repo_model.Repository` the second parameter instead of file
path?
Because then we can easily adapt our repository storage strategy.
The repositories can be stored locally, however, they could just as well
be stored on a remote server.

## Further changes in other PRs
- A Git Command wrapper on package `gitrepo` could be created. i.e.
`NewCommand(ctx, repo_model.Repository, commands...)`. `git.RunOpts{Dir:
repo.RepoPath()}`, the directory should be empty before invoking this
method and it can be filled in the function only. #28940
- Remove the `RepoPath()`/`WikiPath()` functions to reduce the
possibility of mistakes.

---------

Co-authored-by: delvh <dev.lh@web.de>

modules/git/repo_branch_gogit.go

@@ -7,7 +7,6 @@
 package git
 
 import (
-	"context"
 	"sort"
 	"strings"
 
@@ -95,34 +94,6 @@ func (repo *Repository) GetBranchNames(skip, limit int) ([]string, int, error) {
 	return branchNames, len(branchData), nil
 }
 
-// WalkReferences walks all the references from the repository
-// refType should be empty, ObjectTag or ObjectBranch. All other values are equivalent to empty.
-func WalkReferences(ctx context.Context, repoPath string, walkfn func(sha1, refname string) error) (int, error) {
-	repo := RepositoryFromContext(ctx, repoPath)
-	if repo == nil {
-		var err error
-		repo, err = OpenRepository(ctx, repoPath)
-		if err != nil {
-			return 0, err
-		}
-		defer repo.Close()
-	}
-
-	i := 0
-	iter, err := repo.gogitRepo.References()
-	if err != nil {
-		return i, err
-	}
-	defer iter.Close()
-
-	err = iter.ForEach(func(ref *plumbing.Reference) error {
-		err := walkfn(ref.Hash().String(), string(ref.Name()))
-		i++
-		return err
-	})
-	return i, err
-}
-
 // WalkReferences walks all the references from the repository
 func (repo *Repository) WalkReferences(arg ObjectType, skip, limit int, walkfn func(sha1, refname string) error) (int, error) {
 	i := 0