Define diff-hl-command-map without destroying existing bindings
Keymaps are usually defined using `defvar' which has the advantage that
it only sets a variables value if that is not defined yet. As a result
re-evaluating (i.e. during development) a buffer that contains such a
definition does not reset the value, which would lose all user
customization.
Unfortunately `define-prefix-command' does not do that. So use `defvar'
to define the variable and then also store the in the function cell
using `fset'.