test: ensure we can round-trip a written tree
Read a tree into an index, write the index, then re-open the index and
ensure that we are treesame to the original.
round-trip trees through index_read_index
Read a tree into an index using `git_index_read_index` (by reading
a tree into a new index, then reading that index into the current
index), then write the index back out, ensuring that our new index
is treesame to the tree that we read.