git/TreeEntry: LinkTarget simplification (#8323)

See #8222 for context.

This PR removes a call to `Blob.GetBlobContent` and `Blob.DataAsync` and unifies symlink resolution:
- length was unlimited in one case
- length was truncated to 1024 chars in the other case

Now it is hard-limited to 4096 chars (ie error if larger), which is a length which seems appropriate according to https://stackoverflow.com/a/22575737.

### Tests

- Tests are already present in `tests/integration/repo_test.go:972`: `TestRepoFollowSymlink` (it caught a cap/len stupid mistake).
- [x] I did not document these changes and I do not expect someone else to do it.
- [x] I do not want this change to show in the release notes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8323
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

@@ -206,7 +206,7 @@ func GetContents(ctx context.Context, repo *repo_model.Repository, treePath, ref
 	} else if entry.IsLink() {
 		contentsResponse.Type = string(ContentTypeLink)
 		// The target of a symlink file is the content of the file
-		targetFromContent, err := entry.Blob().GetBlobContent(1024)
+		targetFromContent, err := entry.LinkTarget()
 		if err != nil {
 			return nil, err
 		}