I just came up against the same problem.
If you choose not to do the full PATH (environment variable) integration when install git (on Windows) you’ll need to tell IntelliJ where to find git.cmd; you can do this in
Settings > Project Settings > Version Control > VCSs > Git
Example in my case it was portable git looks like this on windows
C:\Users\user\AppData\Local\GitHub\PortableGit_d7effa1a4a322478cd29c826b52a0c118ad3db11\cmd\git.exe