It is easy to add a new empty VC++ shared project (.vcxitems) to the library folder. You can create a new C++ shared project then copy and rename the resulting vcxitems to your library. 

When you have a vcxitems you can click "Add Existing Project" to your solution.

Otherwise, if you don't want to add a vcxitems, then you have manual control to use git for any libraries in the mydocuments\arduino\libraries folder or any other official arduino location. If you add new files to libraries that are in the official locations that are not in the current solution, then click the rescan button in visual micro or restart the ide to detect the new files/libraries.