libgit 0.28.x support

[darix]

are there any changes expected/needed or should it just work?

looks to me the check in extconf.rb was just too pessimistic

[darix]

it breaks with missing git_buf_free. updating to 0.28.1 to fix this issue. more reporting once the building is done.

[darix]

ok i think a rugged release which relaxes the requirement would be enough libgit2 0.28.1 + rugged 0.27.7 work fine for my gitlab.