Get list of modified files from TFVC Provider

[dtretyakov]

We need to have a list of modified files to trigger a personal build.

[rugpanov]

Currently there is only private api to get this list. We should resolve how the extension should get access to this api. There is an instruction how developers can provide a public api: https://code.visualstudio.com/docs/extensionAPI/vscode-api

To get a set of files for commit, message and workItemIds they are using the method: TfvcSCMProvider.GetCheckinInfo(); I think it would convenient to use this method for our extension as well.

[rugpanov]

In the archive there is a patch for tfvc extension. The patch extends extension api to export list of absolute paths of changed files. (The list is required for triggering a personal build via TeamCity extension.) API-for-TC-extension.zip.

We should decide what else information does TeamCity extension require, and sent a request to tfvc extension developers.

[rugpanov]

Implemented at 62d153f