up libgit2
Merge pull request #24 from huanghuang/pygit2:up

up libgit2
git_merge_trees & index.has_conflicts
Fix tree_merge.
Fix tree_merge.
Rollback libgit2.
rebase a little
update Tree_merge PyDoc
add a merge resolve repo to test conflicts
Merge pull request #25 from huanghuang/pygit2:tree_merge

tree_merge & has_conflicts
Merge remote-tracking branch 'upstream/master' into gh_douban