src/vs/editor/common/services/modelService.ts/ModelService/_commonPrefix
class ModelService:	private static _commonPrefix(a: ITextModel, aLen: number, aDelta: number, b: ITextBuffer, bLen: number, bDelta: number): number {
		const maxResult = Math.min(aLen, bLen);

		let result = 0;
		for (let i = 0; i < maxResult && a.getLineContent(aDelta + i) === b.getLineContent(bDelta + i); i++) {
			result++;
		}
		return result;
	}