See #8222 for context.
## git.Blob.NewTruncatedReader
This introduce a new `NewTruncatedReader` method to return a blob-reader which silently truncates when the limit is reached (io.EOF will be returned).
Since the actual size is also returned `GetBlobContent` can pre-allocate a `[]byte` of the full-size (min of the asked size and the actual size) and call `io.ReadFull(rc, buf)` (instead of `util.ReadWithLimit(dataRc, int(limit))` which is convoluted and not used anywhere else).
### Tests
- I added test coverage for Go changes...
- [x] in their respective `*_test.go` for unit tests.
### Documentation
- [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/8223
Reviewed-by: Lucas <sclu1034@noreply.codeberg.org>
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>
