It was frustrating to have have to look up how to rewrite git credentials each time, so here's a script to do such.

git-rewrite-author.sh

#!/bin/bash
#
# 	git-rewrite-author.sh
#
# Change the author credentials in a git repository.  Execute with "--help"
# argument for usage information.

print_usage()
{
	echo "Usage: $(basename $0) <new author> <new email>"
	echo ""
	echo "Uses git-filter-branch (1) to rewrite the author credentials of"
	echo "a git revision history."
}

print_and_exit()
{
	local message="$1"
	local return="$2"

	test -n "$1" && echo "$1" >&2
	test -n "$2" && exit $2 || exit 0
}

rewrite_history()
{
	local author="$1"
	local email="$2"

	git filter-branch -f --commit-filter '
		GIT_COMMITTER_NAME="'"$author"'";
		GIT_AUTHOR_NAME="'"$author"'";
		GIT_COMMITTER_EMAIL="'"$email"'";
		GIT_AUTHOR_EMAIL="'"$email"'";
		git commit-tree "$@";
	' HEAD
}

main()
{
	# Parse --help argument
	test "x$1" == "x--help" && print_and_exit "$(print_usage)"

	# Fail if not enough arguments
	test -n "$2" || print_and_exit "$(print_usage)" 2

	# Test for git repository
	test -d .git || print_and_exit "fatal: not a git repository!" 2

	rewrite_history "$1" "$2"
}
main $@