Update libgit2 source code (324154a)

* Update libgit2 source code to commit
  324154a4538f9e821cc1065b72109033b6d5da03

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Update Makevars.in and Makevars.win

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add argument to git_remote_fetch call

* src/git2r_remote.c (git2r_remote_fetch): Add new argument refspecs.
  Set to NULL to use the base refspec.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>