Git on main thread (#26573)

This moves spawning of the git subprocess to the main thread. We're not
yet
sure why, but when we spawn a process using GCD's background queues,
sub-processes like git-credential-manager fail to open windows.

This seems to be fixable either by using the main thread, or by using a
standard background thread,
but for now we use the main thread.


Release Notes:

- Git: Fix git-credential-manager

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Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>