`git format-patch` includes diffs with rename detection enabled by
default when creating emails.  Match this behavior.

`git format-patch` includes binary diffs by default when creating
emails.  Match this behavior.

Introduce a function to create an email from a diff and multiple inputs
about the source of the diff.

Creating an email from a diff requires many more inputs, and should be
discouraged in favor of building directly from a commit, and is thus in
the `sys` namespace.

Create `git_email_*` which will encapsulate email creation and
application, and `git_email_create_from_commit` in particular, which
creates an email for a single commit.