git-update: Improve error message for non-blob objects
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>