[update-checkout] switch to using a ThreadPool for scheduling git commands

[charles-zablit]

This patch makes update-checkout use a ThreadPoolExecutor instead of a multiprocessing.Pool. The main benefits are:

• Using processes means that the Python modules that make up update-checkout get re-executed every time a subprocess starts. This causes problems when updating the swift repository as the update-checkout files are edited which can cause functions and modules to no longer be available. With threads, the modules are already loaded in memory, we don't rely on the filesystem.
• The git commands are invoked through Popen meaning they are executed in a subprocess. There is no need to open a subprocess in the multiprocessing.Pool to run other subprocesses. A Python thread is a more light weight solution for scheduling tasks.
• There were Timeout errors specifically on Python 3.9 due to CPython not freeing global locks properly (see [update-checkout] force verbose mode if python 3.9 #84825). Using a ThreadPool bypasses that issue meaning that we will be able to reenable the quiet output mode on Python 3.9.

This patch also refactors the shell.run and shell.capture invocations: before, they were relying on pushd and popd with Python context managers. This does not work with multithreading as the shell module is now shared between threads. Now, git commands are executed with git -C /repo_path instead of relying on the working directory.

rdar://162775786

[charles-zablit]

@swift-ci please smoke test

[compnerd]

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[charles-zablit]

@swift-ci please smoke test linux

[charles-zablit]

@swift-ci please smoke test linux

[charles-zablit]

@swift-ci please smoke test linux

[charles-zablit]

@swift-ci please smoke test

[charles-zablit]

@swift-ci please smoke test

[charles-zablit]

@swift-ci please smoke test

[charles-zablit]

@swift-ci please smoke test

[adrian-prantl]

Based on the explanation this sounds like a good change.