I am currently the only developer working on a project I took over from my predecessor. When I took over the project, it was not under source control. So I created a new git
You may try to add old repository as remote repository at your new repository then pull changes to merge, assuming your repositories will be the baseline.
$> git remote add old /path/to/other/gitrepo/.git
# if you want to merge their master branch then pull from it
# otherwise specify the correct branch to merge
$> git pull old master