Reset global variable to NULL after free'ing resource

Fix diff constructor names

Based on feedback from the ObjectiveGit folks, these are some
further updates to diff.h areas that are poorly documented.

The diff constructor functions had some confusing names, where the
"old" side of the diff was coming after the "new" side.  This
reverses the order in the function name to make it less confusing.

Specifically...

* git_diff_index_to_tree becomes git_diff_tree_to_index
* git_diff_workdir_to_index becomes git_diff_index_to_workdir
* git_diff_workdir_to_tree becomes git_diff_tree_to_workdir

Introduce git_blob_is_binary()

Cleanup the empty-clone test correctly

Allow clone to handle empty repos

adding .mailmap file

Help clarify who are the top commiters when using 'shortlog -sn'.

Options structure for git_clone