import a git repository as subtree keeping history

To a git repository, we sometimes want to import another repository, keeping the history.

Let's say there are two repositories like ~/main.git and ~/manual.git. If you want to import manual.git to main.git/doc as a directory, you follows next.

cd ~/main.git
mkdir doc
cd doc

git remote add -f man ~/manual.git
git merge -s ours --no-commit man/master
git read-tree --prefix=doc -u man/master
git pull -s subtree man master
git ci -m "import manual.git"