Ignore zero-padded file modes in fetch and receive-pack

Some repositories such as https://github.com/robbyrussell/oh-my-zsh.git and https://github.com/Modernizr/Modernizr have commits that have extra padding in the file modes. As this is a mostly harmless issue, we add the following flag to ignore this check.

Note that clones already ignore this. Importing from a URL with one of those repositories worked previously, but importing via the Bitbucket or GitHub import mechanism did not.

Relates to #2889 (closed)