Set feed url based on upgrade ring during installer [master]

[jrbriggs]

When upgrading internal installations of VFSForGit (as determined by the explicit presence of upgrade.ring), configure a NuGet feed URL.

Scenarios:

No GVFS installation

2019-02-19 15:33:11.910   GetConfiguredUpgradeRing: Call to gvfs config upgrade.ring failed with Incorrect function
2019-02-19 15:33:11.910   SetNuGetFeedIfNecessary: No upgrade ring configured. Not configuring NuGet feed.

No configured ring

2019-02-19 15:38:13.163   GetConfiguredUpgradeRing: upgrade.ring is 
2019-02-19 15:38:13.163   GetConfiguredUpgradeRing: Unknown upgrade ring: 
2019-02-19 15:38:13.163   SetNuGetFeedIfNecessary: No upgrade ring configured. Not configuring NuGet feed.

Bogus ring

2019-02-19 15:30:05.764   GetConfiguredUpgradeRing: upgrade.ring is bogus
2019-02-19 15:30:05.764   GetConfiguredUpgradeRing: Unknown upgrade ring: bogus
2019-02-19 15:30:05.764   SetNuGetFeedIfNecessary: No upgrade ring configured. Not configuring NuGet feed.

Slow ring

2019-02-19 15:35:08.156   GetConfiguredUpgradeRing: upgrade.ring is slow
2019-02-19 15:35:08.463   SetNuGetFeedIfNecessary: Set upgrade.feedurl to https://pkgs.dev.azure.com/microsoft/_packaging/VFSForGit-Slow/nuget/v3/index.json
2019-02-19 15:35:08.833   SetNuGetFeedIfNecessary: Set upgrade.feedpackagename to Microsoft.VfsForGitEnvironment

[jrbriggs]

Porting from release branch #803