Empty tree in a commit causes a crash

[slarse]

The function that parses trees tries to unpack each line into three values, but empty trees will be pretty empty so there's a crash.

[slarse]

To be clear, this is only a problem if one makes an empty commit when the repo has no content in the previous commit. Can be easily reproduced by creating an empty commit as the first commit:

git init
git commit --allow-empty -m 'Empty commit'  # Creates a commit with an empty root tree