I've slightly modified Peter's answer here to create a reusable, non-interactive shell script called git-split.sh
:
#!/bin/sh
if [[ $# -ne 2 ]] ; then
echo "Usage: git-split.sh original copy"
exit 0
fi
git mv "$1" "$2"
git commit -n -m "Split history $1 to $2 - rename file to target-name"
REV=`git rev-parse HEAD`
git reset --hard HEAD^
git mv "$1" temp
git commit -n -m "Split history $1 to $2 - rename source-file to temp"
git merge $REV
git commit -a -n -m "Split history $1 to $2 - resolve conflict and keep both files"
git mv temp "$1"
git commit -n -m "Split history $1 to $2 - restore name of source-file"