Add bindings for git_graph_* methods

Add graph.go

graph.go

@@ -0,0 +1,36 @@
+package git
+
+/*
+#include <git2.h>
+*/
+import "C"
+import (
+	"runtime"
+)
+
+func (repo *Repository) GraphDescendantOf(commit, ancestor *Oid) (bool, error) {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	ret := C.git_graph_descendant_of(repo.ptr, commit.toC(), ancestor.toC())
+	if ret < 0 {
+		return false, MakeGitError(ret)
+	}
+
+	return (ret > 0), nil
+}
+
+func (repo *Repository) GraphAheadBehind(local, upstream *Oid) (ahead, behind int, err error) {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	var aheadT C.size_t
+	var behindT C.size_t
+
+	ret := C.git_graph_ahead_behind(&aheadT, &behindT, repo.ptr, local.toC(), upstream.toC())
+	if ret < 0 {
+		return 0, 0, MakeGitError(ret)
+	}
+
+	return int(aheadT), int(behindT), nil
+}