How to extract a project from a git repository

The story of how I came into the undocumented git subtree command to help bring back order to an unfortunate
repository.

A little bit of background

It can happen to make wrong choices; in the programming field, in particular, what often happens is that the
consequences arrive suddenly and like an unstoppable escalation. But catastrophisms aside, we come to a need born a few
days ago: in one of the git repositories that I use, which had to act as a "warehouse" for some small utilities, the
situation has exploded. Managing 15 small projects in a single versioning solution is a real nightmare, so I decided to
try to solve the situation, possibly without simply creating new repositories to start from scratch with the commit
history copied into them.

After a little bit of digging, I found a tool which I didn't know anything about: subtree.

Subtree is a script created by extern contributors in the git community and it's bundled in the git installation from
version 1.7.11, but never added to the official documentation (you can, however, read some informations
here). It allows one to create
subtrees, which are branches that will join the ones already existent but in an independent manner, creating something
like a new repository in the repository.

Subtree also offers some commands which enables the execution of really interesting operations: one of them is
split, which will create a new branch that will contain the commit history of a subfolder in particular. That
seems exactly what we need!

Let's suppose that the starting repository has a folder structure like this:

   repository
   │   .git
   │   .gitignore
   │
   └───project1
   │   │   file1_1
   │   │   file1_2
   │
   └───project2
   │   │   file2_1
   │   │   file2_2
   │
   └───project3
   │   │   file3_1
   │   │   file3_2
   │

Now if we wanted to extract the project1 folder we can go like this: first we must create a subtree which will contain
only the commits involving files inside project1

$ git subtree split --prefix=project1 -b split-branch-project1

Then, we can move into another folder and create the future repository

$ mkdir project1
$ cd project1
$ git init

Let's execute a pull from the old repository, referencing the newly created branch:

$ git pull ~/repository split-branch-project1

At this point project1 will contain only the commit history involving that project; by repeating the process for each
project we can have cleaner repositories without losing the history.