No, it doesn't. Unix error codes are extremely well understood. Tell the Git maintainers that their program ignores errors and robustness—they'll be surprised.
Even when well understood, it doesn't mean easy to work with. If you're lucky, the app you're calling only has two states: success+result, or failure+error message. But working with text commands, one day you'll get a "skipped file Xyz" somewhere in the output, because it's neither an error not a success. If you're very lucky, you'll get it in stderr, otherwise it will be mixed with the output. If you're not lucky, the command will print out the error and exit with 0 anyway. What crazy app would do that? For example standard initctl on Ubuntu: https://bugs.launchpad.net/ubuntu/+source/upstart/+bug/55278...
Exit codes are a poor substitute for proper error handling with verbose error reports. They do the job most of the time, as long as you remember exactly which command behaves which way. And that's a clear path to mistakes :-(
Doing things properly in Python or Go may take a few more lines (not much more really) but it is 100 tones more robust, and you need that if you are writing anything more than a 10-line one-off hack.