Rename GitFetchSpecHandle to GitRefSpecHandle
Move Push related tests to a separate file
Add the UpstreamBranchCanonicalName property to Branch

This property contains the reference of the tracked branch, relatively to the upstream repository (so "refs/heads/master", and not "refs/remotes/origin/master").
Add Push overloads taking Branch instances
Add ExplicitPathsOptions to Repository.Diff.Compare()

All the overloads can now report and/or fail upon unmatched explicit paths.
By default, the passed list of paths will be treated as a list of pathspecs.

When an ExplicitPathsOptions is passed to the overloads, this list of paths
will be treated as explicit paths. In that case, the default behavior is to
throw when one of this explicit path is unmatched.
Add ExplicitPathsOptions to Repository.Reset()
Add ExplicitPathsOptions to Repository.Index.Unstage()/Stage()
Guard against marshaling invalid handle as GitOid

Related: https://bugzilla.xamarin.com/show_bug.cgi?id=11417
Move directory-aware Touch() up into BaseFixture
Support additional paths with temp clone
Add submodule test repo
Add Submodule bindings
Add SubmoduleLazyGroup
Add Repository.Submodules

Includes submodule support in Index.Stage() to fix #220
Support retrieving submodule Id in TreeEntry