Move git revision fetching

[zanderso]

Avoids GN invoking python scripts that run git commands to determine git hashes. Instead the git hashes are saved in files by gclient sync, and then read out of the files into GN arguments by the gn script.

Part of flutter/flutter#144430

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Changes reported for pull request #51107 at sha 292a369

[jason-simmons]

Would the performance be acceptable if this invoked the git rev-parse command from within the tools/gn script instead of writing a file during gclient runhooks?

When I run git rev-parse on a repository locally it takes around 30 milliseconds. Doing this within tools/gn will avoid the overhead of launching Python within the build scripts and also avoid the need to keep these temporary files in sync.

[zanderso]

Does git rev-parse hit the network?

[jason-simmons]

No - it should only read the local repository filesystem

[zanderso]

Closing in favor of #51175