NERDTreeCWD: reset CWD if changed by NERDTreeFocus

[PhilRunninger]

Fixes #855.

When the user has 'autochdir' turned on, opening a new NERDTree will
cause the current working directory to change. To prevent this
happening, remember the CWD and reset it if NERDTreeFocus caused it to
change.

[lifecrisis]

This looks like a good start. However, I don't see any care taken to work around the possibility of a window-local current working directory. You would need to use the :lcd command in this case.

It's an edge case. But I think it deserves some investigation.

[lifecrisis]

You should wrap l:cwd with fnameescape() here.

[lifecrisis]

If you read the documentation at :help :NERDTreeCWD, it leads one to believe that our function should be more like this:

function! NERDTreeCWD()
    let l:cwdPath = g:NERDTreePath.New(getcwd())
    call NERDTreeFocus()

    if b:NERDTree.root.path.equals(l:cwdPath)
        return
    endif

    let l:newRoot = g:NERDTreeFileNode.New(l:cwdPath, b:NERDTree)
    call b:NERDTree.changeRoot(l:newRoot)
endfunction

This matches the documentation perfectly, reproduced below:

:NERDTreeCWD                                                  :NERDTreeCWD
    Change tree root to current directory. If no NERD tree exists for this
    tab, a new tree will be opened.

I assert that changing the working directory in any way is not in the scope of this command!

Also, the documentation may be improved a tad...

:NERDTreeCWD                                                  :NERDTreeCWD
    Change the NERDTree root to the current working directory.  If no
    NERDTree exists for this tab, a new one is opened.

EDIT: You can extend this idea to change the working directory of the NERDTree window (if it needs a local one) or to change the global working directory. The important idea here is that the details are saved before we leave the initial context.

EDIT: It should also be pointed out that changing the working directory is pointless when 'autochdir' is set, anyway. Even if we set the directory, leaving and coming back to the NERDTree will just reset it.

[lifecrisis]

I thought about it more last night... I think my above comment and suggested function definition is right on the money. Even in the documentation for 'autochdir', we see:

Note: When this option is on some plugins may not work.

I just don't think the setting is working the way the user thinks it should.

[PhilRunninger]

@lifecrisis , ready for a 2nd review.

[lifecrisis]

Hey @PhilRunninger. Updates look great!

Just added a couple of commits on top of yours. If you're okay with everything, then so am I. Let me know if this is good or if you think of anything else.

[PhilRunninger]

Looks good. I hadn't considered the CD mapping. It's good to have them use the same code.