Replace magic strings with nameof

Author: AArnott

LibGit2Sharp.Shared/Core/GitOdbBackend.cs

@@ -8,7 +8,7 @@ internal struct GitOdbBackend
     {
         static GitOdbBackend()
         {
-            GCHandleOffset = Marshal.OffsetOf(typeof(GitOdbBackend), "GCHandle").ToInt32();
+            GCHandleOffset = Marshal.OffsetOf(typeof(GitOdbBackend), nameof(GCHandle)).ToInt32();
         }
 
         public uint Version;

LibGit2Sharp.Shared/Core/GitOdbBackendStream.cs

@@ -15,7 +15,7 @@ internal class GitOdbBackendStream
     {
         static GitOdbBackendStream()
         {
-            GCHandleOffset = Marshal.OffsetOf(typeof(GitOdbBackendStream), "GCHandle").ToInt32();
+            GCHandleOffset = Marshal.OffsetOf(typeof(GitOdbBackendStream), nameof(GCHandle)).ToInt32();
         }
 
         public IntPtr Backend;

LibGit2Sharp.Shared/Core/GitSmartSubtransport.cs

@@ -8,7 +8,7 @@ internal class GitSmartSubtransport
     {
         static GitSmartSubtransport()
         {
-            GCHandleOffset = Marshal.OffsetOf(typeof(GitSmartSubtransport), "GCHandle").ToInt32();
+            GCHandleOffset = Marshal.OffsetOf(typeof(GitSmartSubtransport), nameof(GCHandle)).ToInt32();
         }
 
         public action_callback Action;

LibGit2Sharp.Shared/Core/GitSmartSubtransportStream.cs

@@ -8,7 +8,7 @@ internal class GitSmartSubtransportStream
     {
         static GitSmartSubtransportStream()
         {
-            GCHandleOffset = Marshal.OffsetOf(typeof(GitSmartSubtransportStream), "GCHandle").ToInt32();
+            GCHandleOffset = Marshal.OffsetOf(typeof(GitSmartSubtransportStream), nameof(GCHandle)).ToInt32();
         }
 
         public IntPtr SmartTransport;