Make dotnet/coreclr reference corresponding dotnet/corefx branch

netci.groovy

@@ -1588,7 +1588,7 @@ combinedScenarios.each { scenario ->
                                                 // thinks that %workspace% is the project base directory.
                                                 buildCommands += "powershell new-item clr -type directory -force"
                                                 buildCommands += 'powershell foreach ($x in get-childitem -force) { if (\$x.name -ne \'clr\') { move-item $x clr }}'
-                                                buildCommands += "git clone https://github.com/dotnet/corefx fx"
+                                                buildCommands += "git clone -b $branch --single-branch https://github.com/dotnet/corefx fx"
 
                                                 buildCommands += getStressModeEnvSetCmd(os, scenario);
 
@@ -1729,7 +1729,7 @@ combinedScenarios.each { scenario ->
                                         buildCommands += "rm -rf .clr; mkdir .clr; mv * .clr; mv .git .clr; mv .clr clr"
 
                                         // Get corefx
-                                        buildCommands += "git clone https://github.com/dotnet/corefx fx"
+                                        buildCommands += "git clone -b $branch --single-branch https://github.com/dotnet/corefx fx"
 
                                         // Set environment variable
                                         def setEnvVar = getStressModeEnvSetCmd(os, scenario)