Git Files Encryption for Business
There really is no way to take on your problem with git, which is to deny a developer with access to the code (required to work with it) the possibility to steal the code. This is because as soon as your developer has the code in a readable format (to program on it), he can do with it whatever he wants, anyways.

git-crypt has a totally different use, and is not related to your problem at all.

To solve your problem, there is basically only one possibility: lock the network in which the code resides down completely - make it so that there is no physical way to transfer the code to the outside. This could mean:


No direct internet connection, obviously. 
No http/https proxy either, no mail clients etc.
No way to add new devices to the network (unless through official ways). E.g., no unused network ports to plug anything in; no guest DHCP; MAC address filtering; no WIFI, smartphones; etc.
No way to plug an USB device or other transfer mechanism into any devices.


This may sound harsh, but really is not. It could be as simple as keeping your code strictly within a set of VMs, accessed by something like Vmware Horizon or something like that, with cut&amp;paste and file transfer disabled, of course. Be nice to your developers and allow them to transfer stuff into the environment (their favourite editor, for example).

The measures so far do not require any physical changes, i.e. your developers still can work remotely or in their usual unsecure office. Obviously, they still can go to extreme measures (screenshots on their client PC...) to get specific parts of the code page-by-page.

To go a step further, in increasing severity (although we are going into James Bond territory here...): 


Strict policy with severe punishment. Setup a specific contract which each emplyee which contains clauses as harsh as the laws of your country allow.
Switching access to the limited network to a physical area in your office.
Security measures like locked doors accessible to only specific personnel, video cameras with 24/7 human supervision and so on. 
2-factor access (either 2 persons required to checkout code, or at least a physical TAN generator or something along those lines).


Note that in your case you want to make it hard for a thief who knows that he is doing something illegal and already has committed to doing a crime. He will not respect any "weak" protection (like rules/laws) anyways.