A TREE extension with an entry count of -1 means that it was
invalidated and we should ignore it. Do so instead of returning an
error.

This fixes issue #202

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

There are two reasons why read_tree_internal might return a NULL
tree. The first one is a corrupt index, but the second one is an
invalidated TREE extension. Up to now, its only way to communicate
with its caller was through the return value being NULL or not.

Allow read_tree_internal to report its exit status independently from
the tree pointer.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

The `git_repository_index` call now returns a brand new index that must
be manually free'd.

Fix compilation warnings in MSVC

This allows to successfully build libgit2 with waf on Windows.

When trying to inflate a buffer, a GIT_ERROR was returned when malloc()
failed.

Fix this to return GIT_ENOMEM.