git/blob: GetContentBase64 with fewer allocations and no goroutine (#8297)

See #8222 for context.i

`GetBlobContentBase64` was using a pipe and a goroutine to read the blob content as base64. This can be replace by a pre-allocated buffer and a direct copy.

Note that although similar to `GetBlobContent`, it does not truncate the content if the blob size is over the limit (but returns an error). I think that `GetBlobContent` should adopt the same behavior at some point (error instead of truncating).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8297
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

services/repository/files/content.go

@@ -5,6 +5,7 @@ package files
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net/url"
 	"path"
@@ -273,13 +274,11 @@ func GetBlobBySHA(ctx context.Context, repo *repo_model.Repository, gitRepo *git
 	if err != nil {
 		return nil, err
 	}
-	content := ""
-	if gitBlob.Size() <= setting.API.DefaultMaxBlobSize {
-		content, err = gitBlob.GetBlobContentBase64()
-		if err != nil {
-			return nil, err
-		}
+	content, err := gitBlob.GetContentBase64(setting.API.DefaultMaxBlobSize)
+	if err != nil && !errors.As(err, &git.BlobTooLargeError{}) {
+		return nil, err
 	}
+
 	return &api.GitBlob{
 		SHA:      gitBlob.ID.String(),
 		URL:      repo.APIURL() + "/git/blobs/" + url.PathEscape(gitBlob.ID.String()),