fix #24 - support both branch refs and hash refs

actions/testdata/gitref.workflow

@@ -0,0 +1,14 @@
+workflow "New workflow" {
+  on = "push"
+  resolves = ["branch-ref","commit-ref"]
+}
+
+action "branch-ref" {
+  uses = "actions/docker/cli@master"
+  args = "version"
+}
+
+action "commit-ref" {
+  uses = "actions/docker/cli@c08a5fc9e0286844156fefff2c141072048141f6"
+  args = "version"
+}