Add a way to cleanly shut down the library (#578)

This change adds the Shutdown() method, so that the library can be
cleanly shut down. This helps significanly reduce the amount of noise in
the leak detector.

(cherry picked from commit 619a9c2)

Author: lhchavez

git.go

@@ -139,6 +139,16 @@ func init() {
 	C.git_openssl_set_locking()
 }
 
+// Shutdown frees all the resources acquired by libgit2. Make sure no
+// references to any git2go objects are live before calling this.
+// After this is called, invoking any function from this library will result in
+// undefined behavior, so make sure this is called carefully.
+func Shutdown() {
+	pointerHandles.Clear()
+
+	C.git_libgit2_shutdown()
+}
+
 // Oid represents the id for a Git object.
 type Oid [20]byte
 

handles.go

@@ -43,6 +43,16 @@ func (v *HandleList) Untrack(handle unsafe.Pointer) {
 	v.Unlock()
 }
 
+// Clear stops tracking all the managed pointers.
+func (v *HandleList) Clear() {
+	v.Lock()
+	for handle := range v.handles {
+		delete(v.handles, handle)
+		C.free(handle)
+	}
+	v.Unlock()
+}
+
 // Get retrieves the pointer from the given handle
 func (v *HandleList) Get(handle unsafe.Pointer) interface{} {
 	v.RLock()