Update libgit2 source code (f54d8d52)

* Update libgit2 source code to commit
  f54d8d528a5843075c6c60167d4659e393956fa3

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

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