Eagerly dispose of Remotes

[niik]

This is a continuation of #1249. Since Remotes are now disposable we should eagerly clean up their resources rather than wait for GC to call
the destructor.

cc @ethomson @carlosmn

[niik]

See #1249 (comment)

[niik]

There's also a load of tests which use Remote that I didn't touch here

[carlosmn]

We could add

diff --git a/LibGit2Sharp/Remote.cs b/LibGit2Sharp/Remote.cs
index f92b4aa..a318707 100644
--- a/LibGit2Sharp/Remote.cs
+++ b/LibGit2Sharp/Remote.cs
@@ -39,6 +39,7 @@ internal Remote(RemoteSafeHandle handle, Repository repository)
             PushUrl = Proxy.git_remote_pushurl(handle);
             TagFetchMode = Proxy.git_remote_autotag(handle);
             refSpecs = new RefSpecCollection(this, handle);
+            repository.RegisterForCleanup(this);
         }

         ~Remote()

which makes us release the handle when the repository itself gets disposed.