feat(turborepo): add `get_merge_base` to turborepo_scm #1719

Job	Run time
Determine jobs to run	20s
Check next-swc	0s
Turbopack rust check	0s
Turbopack TypeScript files	0s
Turbopack Rust testing on ${{ matrix.os.name }}	0s
Turbopack Rust testing benchmarks on ${{ matrix.os.name }}	0s
Turbopack rust clippy	0s
Turbopack Rust testing benchmarks on linux	0s
Turbopack Rust testing on ubuntu	0s
Upload Test results	54s
Ok	4s
Done	3s
Cleanup / cleanup	1s
	1m 22s

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