Fix file modes
Remove dependency on File::Slurp from some modules
Implement Git::More::get_head_or_empty_tree routine
Fix Git::More::filter_files_in_index

When it is invoked on a brand new repository HEAD isn't defined and we
have to diff the index against the empty tree.
Fix whitespace
CheckWhitespace: implement this new plugin

This plugin uses 'git diff --check' to detect whitespace errors in the
files being changed.
v1.5.0

  [New features]

  - New plugin CheckWhitespace to detect whitespace errors in files.

  - New routine Git::More::get_head_or_empty_tree returns the SHA1
    representing the empty tree when HEAD is undefined, which happens in a
    brand new repository.

  [Fixes]

  - Fix Git::More::filter_files_in_index to work in brand new repositories.