0.1.1: Use empty tree as base revision if HEAD^ invalid

In certain instances (such as creating a repository from a template in
which existing history is collapsed), HEAD^ may not refer to a valid
revision because the initial commit has no parent. Consequently, the
action fails with the following error message:

    fatal: ambiguous argument 'HEAD^': unknown revision or path not
            in the working tree.

This change verifies that the base revision is valid before attempting
to use the reference in the invocation of `git diff`. If HEAD^ does
not exist, then the revision of the empty tree is used as the base
instead -- i.e., an empty tree represents the (hypothetical) parent of
the current commit.

Closes #2

Initial release

0.1

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