Only preload .user file when it exists

Don't attempt to preload GitHub.VisualStudio.csproj.user if the user
hasn't created one.

Author: jcansdale

src/GitHub.VisualStudio/GitHub.VisualStudio.csproj

@@ -2,8 +2,8 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="14.0">
   <Import Project="..\..\packages\LibGit2Sharp.NativeBinaries.1.0.164\build\LibGit2Sharp.NativeBinaries.props" Condition="Exists('..\..\packages\LibGit2Sharp.NativeBinaries.1.0.164\build\LibGit2Sharp.NativeBinaries.props')" />
 
-  <!-- When StartProgram is set to Visual Studio 2015, install extension in this version  -->
-  <Import Project="GitHub.VisualStudio.csproj.user" />
+  <!-- When StartProgram is set to Visual Studio 2015 in .user file, install extension in the same version  -->
+  <Import Project="GitHub.VisualStudio.csproj.user" Condition=" Exists('GitHub.VisualStudio.csproj.user') " />
   <PropertyGroup Condition=" $(StartProgram.Contains('Microsoft Visual Studio 14.0')) ">
     <VisualStudioVersion>14.0</VisualStudioVersion>
   </PropertyGroup>