Add support for create source git with --commit and --ref-name flags

Signed-off-by: Santosh Kaluskar <dtshbl@gmail.com>

cmd/flux/testdata/create_source_git/source-git-refname.yaml

@@ -0,0 +1,11 @@
+---
+apiVersion: source.toolkit.fluxcd.io/v1beta2
+kind: GitRepository
+metadata:
+  name: podinfo
+  namespace: flux-system
+spec:
+  interval: 1m0s
+  ref:
+    name: refs/heads/main
+  url: https://github.com/stefanprodan/podinfo