this commit is a modified patch from Guillaume Duranceau

this commit is a modified patch from Guillaume Duranceau

this way we can e.g. not show the git submenu unless a git repo is
actually present

previously the code didnt take &hidden (it assumed it was set) into
account when determining if splitting was needed when using the o
mapping on a file node

Previously every time the tree window was created a new [no-name] buffer
was being created (visible with :ls!). Avoid this by creating the window
with split,  not new