Upgrade LibGit2Sharp to 0.26.0-preview-0070

From ethomson:

> I've released a prerelease LibGit2Sharp that should help a bit here. I've
> updated libgit2 to have built-in support for proxies, so we no longer rely
> upon libcurl. I think this should help the native dependency problems quite
> a bit.

This should resolve the libcurl4 issue with have on Ubuntu 18.04.1 (#13).

src/git-istage/git-istage.csproj

@@ -15,7 +15,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="LibGit2Sharp" Version="0.25.3" />
+    <PackageReference Include="LibGit2Sharp" Version="0.26.0-preview-0070" />
     <PackageReference Include="Microsoft.DotNet.Analyzers.Compatibility" Version="0.2.12-alpha">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>