Use readline for reading git commit messages

This way, you can use the arrow keys, etc. to edit input, and they work, rather than outputting control sequences.

I believe this is the only instance of reading user input where readline is useful.
On line 115 below (originally 105),

    typeset comment="${4:-}"

I noticed you provide an empty default here. Is this not equivalent since `$4` is blank?

    typeset comment="$4"

If there is some nuance I am unaware of, I should also change this line to match that style.