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

this is because of vims fucking stupid functionality where ignorecase
changes the way == behaves. It was causing bugs when the user had
ignorecase set.

now we can go   vim  <some dir>   and it loads up a nerd tree instead of
a netrw

when the user activate the   ../ (up a dir)   line or hit U or U it
wasnt changing vims CWD when NERDTreeChDirMode==2

allows the statusline to be customised for the nerd tree window

Path.New was expecting an absolute path. Now we convert paths to
absolute inside Path.New

make sure we always go back to the right buffer, and if there is no
buffer to go back to, close the window