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_nogogit.go

@@ -65,11 +65,6 @@ func (repo *Repository) GetBranchNames(skip, limit int) ([]string, int, error) {
 	return callShowRef(repo.Ctx, repo.Path, BranchPrefix, TrustedCmdArgs{BranchPrefix, "--sort=-committerdate"}, skip, limit)
 }
 
-// WalkReferences walks all the references from the repository
-func WalkReferences(ctx context.Context, repoPath string, walkfn func(sha1, refname string) error) (int, error) {
-	return walkShowRef(ctx, repoPath, nil, 0, 0, walkfn)
-}
-
 // WalkReferences walks all the references from the repository
 // refType should be empty, ObjectTag or ObjectBranch. All other values are equivalent to empty.
 func (repo *Repository) WalkReferences(refType ObjectType, skip, limit int, walkfn func(sha1, refname string) error) (int, error) {
@@ -81,12 +76,12 @@ func (repo *Repository) WalkReferences(refType ObjectType, skip, limit int, walk
 		args = TrustedCmdArgs{BranchPrefix, "--sort=-committerdate"}
 	}
 
-	return walkShowRef(repo.Ctx, repo.Path, args, skip, limit, walkfn)
+	return WalkShowRef(repo.Ctx, repo.Path, args, skip, limit, walkfn)
 }
 
 // callShowRef return refs, if limit = 0 it will not limit
 func callShowRef(ctx context.Context, repoPath, trimPrefix string, extraArgs TrustedCmdArgs, skip, limit int) (branchNames []string, countAll int, err error) {
-	countAll, err = walkShowRef(ctx, repoPath, extraArgs, skip, limit, func(_, branchName string) error {
+	countAll, err = WalkShowRef(ctx, repoPath, extraArgs, skip, limit, func(_, branchName string) error {
 		branchName = strings.TrimPrefix(branchName, trimPrefix)
 		branchNames = append(branchNames, branchName)
 
@@ -95,7 +90,7 @@ func callShowRef(ctx context.Context, repoPath, trimPrefix string, extraArgs Tru
 	return branchNames, countAll, err
 }
 
-func walkShowRef(ctx context.Context, repoPath string, extraArgs TrustedCmdArgs, skip, limit int, walkfn func(sha1, refname string) error) (countAll int, err error) {
+func WalkShowRef(ctx context.Context, repoPath string, extraArgs TrustedCmdArgs, skip, limit int, walkfn func(sha1, refname string) error) (countAll int, err error) {
 	stdoutReader, stdoutWriter := io.Pipe()
 	defer func() {
 		_ = stdoutReader.Close()
@@ -189,7 +184,7 @@ func walkShowRef(ctx context.Context, repoPath string, extraArgs TrustedCmdArgs,
 // GetRefsBySha returns all references filtered with prefix that belong to a sha commit hash
 func (repo *Repository) GetRefsBySha(sha, prefix string) ([]string, error) {
 	var revList []string
-	_, err := walkShowRef(repo.Ctx, repo.Path, nil, 0, 0, func(walkSha, refname string) error {
+	_, err := WalkShowRef(repo.Ctx, repo.Path, nil, 0, 0, func(walkSha, refname string) error {
 		if walkSha == sha && strings.HasPrefix(refname, prefix) {
 			revList = append(revList, refname)
 		}