I guess "Reviewed by: imp" shouldn't be hard-coded? :)

also perhaps some commentary on $HOME/bin/git-fixup-editor

ah, yea, likely shouldn't be hard coded.
And git-fixup=editor likely should be included here as well.

#!/bin/sh

# Takes 1 arg, the file to edit

fn=$1
dir=$(mktemp -d gfe-XXXXXXXXXXXX)
ofn=$dir/msg

trap "rm -rf $dir" EXIT

sed -e 's/^Reviewed-by:/Reviewed by:/;
        s/^Pull-Request:/Pull Request:/;
        s/^Differential-Revision:/Differential Revision:/' < $fn > $ofn && mv $ofn $fn

is what it does.

This gets back to a point I'd like to raise: We should ditch the 'metadata with spaces' thing and move to more-standard metadata so that the git commit --trailer stuff can be used in the tooling.