src/vs/workbench/services/workingCopy/common/workingCopyBackupService.ts/WorkingCopyBackupServiceImpl/readToMatchingString
class WorkingCopyBackupServiceImpl:	private async readToMatchingString(backupResource: URI, matchingString: string, maximumBytesToRead: number): Promise<string | undefined> {
		const contents = (await this.fileService.readFile(backupResource, { length: maximumBytesToRead })).value.toString();

		const matchingStringIndex = contents.indexOf(matchingString);
		if (matchingStringIndex >= 0) {
			return contents.substr(0, matchingStringIndex);
		}

		// Unable to find matching string in file
		return undefined;
	}