Fixes the reason why go-gitea#29101 is hard to replicate.
Related go-gitea#29297

Create a repo with a file with minimum size 4097 bytes (I use 10000) and
execute the following code:
```go
gitRepo, err := gitrepo.OpenRepository(db.DefaultContext, <repo>)
assert.NoError(t, err)

commit, err := gitRepo.GetCommit(<sha>)
assert.NoError(t, err)

entry, err := commit.GetTreeEntryByPath(<file>)
assert.NoError(t, err)

b := entry.Blob()

// Create a reader
r, err := b.DataAsync()
assert.NoError(t, err)
defer r.Close()

// Create a second reader
r2, err := b.DataAsync()
assert.NoError(t, err) // Should be no error but is ErrNotExist
defer r2.Close()
```

The problem is the check in `CatFileBatch`:

https://github.com/go-gitea/gitea/blob/79217ea63c1f77de7ca79813ae45950724e63d02/modules/git/repo_base_nogogit.go#L81-L87
`Buffered() > 0` is used to check if there is a "operation" in progress
at the moment. This is a problem because we can't control the internal
buffer in the `bufio.Reader`. The code above demonstrates a sequence
which initiates an operation for which the code thinks there is no
active processing. The second call to `DataAsync()` therefore reuses the
existing instances instead of creating a new batch reader.