MANIFEST.SKIP: fix
v0.29

 - Update to libgit2 v0.20
   + Remove Branch -> foreach() (incompatible change)
   + Repository -> branches() now returns a "real" list instead of an array
     reference
 - Rename Cred -> plaintext() to Cred -> userpass() and Cred -> keyfile()
   to Cred -> sshkey() (incompatible change)
 - Add Cred -> sshagent() method
 - Add is_tree() and is_blob() to Tree and Blob (GH#22) (thanks, @ap!)