Update libgit2 to development branch
Add diff support
Add better remotes support (add, rename, remove)
Merge pull request #34 from metacran/feature-remotes

Add better remote support (add, rename, remove)
Resolved conflicts Makevars when merge diff support
Merge branch 'metacran-feature-diff'
Remove unused variables
Change show output from class git_commit to be more compact

* R/commit.r (show): Changed the output over several lines to a
  one-liner with the following layout:
  [6b91a4f] 2014-05-28: Remove unused variables
Add is.merge to determine if a commit has more than one parent
Improve git_commit summary output to include diff and merge info
Added sort args (topological, time, reverse) to commits method

* R/commit.r (commits): Added sort arguments; topological, time and
  reverese to determine the sort order of the commits in the
  result. Default is topological = TRUE, time = TRUE and reverse =
  FALSE.

* src/git2r_revwalk.c/h (git2r_revwalk_list): Added support for the
  sort arguments.

* src/git2r.c: Updated number of arguments of git2r_revwalk_list in
  callMethods.

* man/commits-methods.Rd: Updated documentation.