Use comint-mode-map as parent map instead

[Inc0n]

Whilst playing with inf-clojure-mode-map, some previous binds are overriden, because copy-keymap is used instead of set-parent-map. This will help for that.

• The commits are consistent with our contribution guidelines
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the readme (if adding/changing user-visible functionality)

[bbatsov]

Thanks!

[Inc0n]

A small change, but a convenient one :)