CRAN release v0.21.0

• Added methods 'odb_blobs' and 'odb_objects' with missing repository
  signature. Internally, they use 'getwd' and 'discover_repository' to
  open a repository.

BUG FIXES

• The bundled libgit2 source code has been reverted to libgit2 v0.26.0
  (15e1193) from 14 June 2017 (same as in git2r v0.19.0) to fix memory
  alignment errors.