/usr/bin/git-graft is in git-extras 1.9.1-2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | #!/bin/sh
src=$1
dst=$2
test -z $src && echo "source branch required." 1>&2 && exit 1
git checkout $dst \
&& git merge --no-ff $src \
&& git branch -d $src
|