[No QA] Make GitUtils use spawn/stream instead of execSync/buffer

[roryabraham]

Details

This fixes an error we've never seen before with our deploy process: https://expensify.slack.com/archives/C03V9A4TB/p1657736346352819

The problem is that execSync uses a buffer with a 200kb limit, and if the command's output exceeds that amount then it will 🤮

This solution uses spawn, which uses an output stream instead of a buffer, so we don't have to worry about running out of buffer space. Got the solution from https://stackoverflow.com/questions/63796633/spawnsync-bin-sh-enobufs

Alternate solution: Use increase the buffer size. But then there's no telling if we'd run into this problem in the future if we had a larger commit volume / more time between deploys.

Fixed Issues

$ n/a – broken deploys

Tests

0. Automated tests using this function are updated and passing.
1. Merge this PR
2. Verify that it triggers a deploy.

[roryabraham]

Additional context with the .mjs / await weirdness:

https://expensify.slack.com/archives/C01GTK53T8Q/p1657745375254589

[roryabraham]

Monitoring here https://github.com/Expensify/App/actions/runs/2666787731

[OSBotify]

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