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>

routers/api/v1/repo/wiki.go

@@ -5,6 +5,7 @@ package repo
 
 import (
 	"encoding/base64"
+	"errors"
 	"fmt"
 	"net/http"
 	"net/url"
@@ -506,11 +507,8 @@ func findWikiRepoCommit(ctx *context.APIContext) (*git.Repository, *git.Commit)
 // given tree entry, encoded with base64. Writes to ctx if an error occurs.
 func wikiContentsByEntry(ctx *context.APIContext, entry *git.TreeEntry) string {
 	blob := entry.Blob()
-	if blob.Size() > setting.API.DefaultMaxBlobSize {
-		return ""
-	}
-	content, err := blob.GetBlobContentBase64()
-	if err != nil {
+	content, err := blob.GetContentBase64(setting.API.DefaultMaxBlobSize)
+	if err != nil && !errors.As(err, &git.BlobTooLargeError{}) {
 		ctx.Error(http.StatusInternalServerError, "GetBlobContentBase64", err)
 		return ""
 	}