rename head_orphan? -> head_unborn? into README.md
Merge pull request #399 from daisuko/fix-rename-method-into-readme

update README
Bump libgit2 to 59403f1ff55346c64bfaa0744ea7f3375da71725
Type check the tree value for Rugged::Index#read_tree

libgit2 does no type checking in `git_index_read_tree` which means that
`index.read_tree` will segfault if you pass something other than a tree.

This ensures that the value passed is actually a tree.
Tabs not spaces
Merge pull request #401 from libgit2/type-check-read-tree

Type check the tree value for Rugged::Index#read_tree
Update Changelog for #401.
Update the Changelog for `Rugged::Remote#check_connection`.
Update the changelog for `Rugged::Repository#expand_oids`.