Prevent GenerateGitVersionInformation in XAML generated projects

During XAML'S build magic a temporary project may get generated and
build (target "GenerateTemporaryTargetAssembly"). The generated project
file already contains compile-items with the generated files from the
original project, this `GitVerstionInformation.g.cs` is listed. When the
GitVersion task now runs again, the file is added another time resulting
in an error of CSC.
This patch tries to detect this situation and prevent the execution of
GenerateGitVersionInformation by checking for a special property
injected by GenerateTemporaryTargetAssembly. Unfortunately this is not
documented and thus may change.

See also
- https://referencesource.microsoft.com/#PresentationBuildTasks/BuildTasks/Microsoft/Build/Tasks/Windows/GenerateTemporaryTargetAssembly.cs,163
- #2228

Author: jbaehr

src/GitVersionTask/build/GitVersionTask.props

@@ -24,6 +24,14 @@
 
         <!-- Property that enables GenerateGitVersionInformation -->
         <GenerateGitVersionInformation Condition=" '$(DisableGitVersionTask)' == 'true' ">false</GenerateGitVersionInformation>
+        <!--
+          During XAML's "GenerateTemporaryTargetAssembly" a new project file is generated and compiled
+          which already contains the "GeneratedCodeFiles", i.e. GitVersionInformation.g.cs.
+          Then, when GenerateGitVersionInformation is called in this temp-build, the file is added another time, which results in an error at CSC.
+          Here we try to detect this situation and prevent GenerateGitVersionInformation from running.
+          (The global property "_TargetAssemblyProjectName" is injected by XAML's above-mentioned task)
+        -->
+        <GenerateGitVersionInformation Condition=" '$(GenerateGitVersionInformation)' == '' And '$(_TargetAssemblyProjectName)' != '' ">false</GenerateGitVersionInformation>
         <GenerateGitVersionInformation Condition=" '$(GenerateGitVersionInformation)' == '' ">true</GenerateGitVersionInformation>
 
         <!-- Property that enables GetVersion -->