Use file service to match string in the file

[sandy081]

Back up service uses following util to read and match the content in the file.

vscode/src/vs/base/node/stream.ts

Line 17 in 60dcb1a

export function readToMatchingString(file: string, matchingString: string, chunkBytes: number, maximumBytesToRead: number): Promise<string | null> {

This uses the node fs.

Please use IFileService api which is capable of reading bytes and streams.