Improve GenerateAssemblyCommitIdsCs

Makes it handle being run when the project is not in a git repo.

Author: bording

LibGit2Sharp/CodeGenerator.targets

@@ -69,16 +69,18 @@
   </Target>
 
 
-  <Target Name="GenerateAssemblyCommitIdsCs" Inputs="$(libgit2_propsfile);$(VersionSourceFile)" Outputs="$(AssemblyCommitIdsPath)" BeforeTargets="CoreCompile" DependsOnTargets="DefineProperties;GenerateAssemblyVersionInfo" Condition="'$(GitCommitId)' != ''">
+  <Target Name="GenerateAssemblyCommitIdsCs" Inputs="$(libgit2_propsfile);$(VersionSourceFile)" Outputs="$(AssemblyCommitIdsPath)" BeforeTargets="CoreCompile" AfterTargets="GenerateAssemblyVersionInfo" DependsOnTargets="DefineProperties">
 
     <PropertyGroup>
+      <LibGit2SharpCommitSha>unknown</LibGit2SharpCommitSha>
+      <LibGit2SharpCommitSha Condition="'$(GitCommitId)' != ''">$(GitCommitId)</LibGit2SharpCommitSha>
       <AssemblyCommitIdsSourceLines>
         namespace LibGit2Sharp
         {
         internal static class AssemblyCommitIds
         {
         public const string LibGit2CommitSha = "$(libgit2_hash)"%3b
-        public const string LibGit2SharpCommitSha = "$(GitCommitId)"%3b
+        public const string LibGit2SharpCommitSha = "$(LibGit2SharpCommitSha)"%3b
         }
         }
       </AssemblyCommitIdsSourceLines>