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()
Allow update of the target of a symbolic reference with another symbolic reference
Introduce Repository.Refs.Log()

returns the reflog of the reference given as parameter
Insert new reflog entry on commit

Fix #371

Insert entry on HEAD and target direct reference on commit
Tag as initial the first commit
Small code refactoring
Make amend on orphaned head throw OrphanedHeadException
Enforce reflog writing test coverage
Test reflog writing when HEAD is detached
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