You will need to create a GITDIR variable which point to the root of all git repo.
(If you don't know how, you type cd ~, then modify/create a file named .profile and add export GITDIR="..." in it.)
There are explanations below for all functions.
Use update_git (without arguments) to update all repositories (and sub-repo) found in GITDIR, whereas use update_git repo1 repo2 ... to update only them.
/!\ The root is considered to be GITDIR, if you want to update a project that is in a subdirectory of GITDIR or one that is 'next' to it, you need to use the path relative to GITDIR.
When using clean_git, the program will loop through each folder and use git gc --agressive every time, along with showing the size evolution.
Use make install as root to install the file in the /usr/bin folder as update_git.
To customize either the name or the directory to install, you have to modify the Makefile.