When a repository contains a directory, a user is currently faced with
the following error message:

    remote: Traceback (most recent call last):
    remote:   File "hooks/update", line 194, in <module>
    remote:     if repo[treeobj.id].size > 250000:
    remote: AttributeError: '_pygit2.Tree' object has no attribute 'size'
    remote: error: hook declined to update refs/heads/master

Explicitly check for directories and other non-blob objects and display
a more intuitive message.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>