Prevent double use of git cat-file session.

[KN4CK3R]

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

Create a repo with a file with minimum size 4097 bytes (I use 10000) and execute the following code:

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:

gitea/modules/git/repo_base_nogogit.go

Lines 81 to 87 in 79217ea

	func (repo *Repository) CatFileBatch(ctx context.Context) (WriteCloserError, *bufio.Reader, func()) {
	if repo.batchCancel == nil || repo.batchReader.Buffered() > 0 {
	log.Debug("Opening temporary cat file batch for: %s", repo.Path)
	return CatFileBatch(ctx, repo.Path)
	}
	return repo.batchWriter, repo.batchReader, func() {}
	}

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.

[KN4CK3R]

No atomic because this is single thread code.

[GiteaBot]

I was unable to create a backport for 1.21. @KN4CK3R, please send one manually. 🍵

go run ./contrib/backport 29298
...  // fix git conflicts if any
go run ./contrib/backport --continue

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