The current behaviour of git_index_open{bare,inrepo}() is unexpected.
When an index is opened, an in-memory index object is created that is
linked to the index discovered by git_repository_open(). However, this
index object is empty, as the on-disk index is not read. To fully open
the on-disk index file, git_index_read() has to be called. This leads to
confusing behaviour. Consider the following code:

	git_index *idx;
	git_index_open_inrepo(&idx, repo);
	git_index_write(idx);

You would expect this to have no effect, as the index is never
ostensibly manipulated. However, what actually happens is that the index
entries are removed from the on-disk index because the empty in-memory
index object created by open_inrepo() is written back to the disk.

This patch reads the index after opening it.