Restrict visibility of Gitflow menu items
* "Init Gitflow" only visible on the repository node (similar to "enable
LFS" and "Add Submodule", which are all a kind of configuration change)
* Gitflow sub menu only visible on the repository node, the "branches"
node and all children of the "branches" node.
The change is most notable when opening the context menu on tags, the
working tree etc.
Bug: 536205
Change-Id: Icd69482f2c2415444c22a1f6690fcc589f05a25a
Signed-off-by: Michael Keppler <Michael.Keppler@gmx.de>