Edit and Continue thinks there's edits made to a project if you open 2 instances of the same project

[davkean]

Version Used: Version 15.3.0 Preview 4.0 [26629.0.d15rel]

Steps to Reproduce:

1. Clone http://github.com/davkean/roslyn-project-system
2. Sync to f4af53786c3ab95d79683d45681db5d587a96a96
3. Open ProjectSystem.sln
4. Open src\Microsoft.VisualStudio.ProjectSystem.Managed.VS\ProjectSystem\VS\IProjectTreeExtensions.cs
5. Set a breakpoint on line 100
6. Set ProjectSystemSetup as the default project
7. F5
8. In the new instance of VS open ProjectSystem.sln
9. Wait for breakpoint to be hit
10. Press F5 to continue

Expected Behavior: For debugging to continue

Actual Behavior:

[jcouv]

FYI @tmat