Vim: switch NERDTree submodule URL to development repo

This one has seen some activity in the last few years, unlike the other
one.

Changes since 4.2.0 (list produced manually with `git log --oneline
--no-decorate --no-merges 4.2.0..master` because the SHA-1 of the
4.2.0 release is different in these two repos):

> Truncate paths at slashes.
> easier to read one of the FAQ answers
> use the built in fnamemodify() to make a path relative
> fix escaping for `:edit` when renaming files
> apply @mixvin's windows path fix at the top of the method instead
> Fix opening file path, in Windows
> Fix reloading renamed file
> Fix doc map for NERDTreeMapChangeRoot
> add refreshDirFlags method to avoid refreshing too many files
> move nerdtree#treeExists.* methods into the NERDTree class
> add proper events and make the notifier class generic
> allow plugins to override the default mappings
> fix a bug from recent refactoring RE some NERDTreeCWD
> add highlighting for flags
> remove testing git plugin
> move some more view code into the UI class
> Use event BufLeave instead of BufWinLeave to record screen state to avoid undefined b:NERDTree error
> fix the NERDTreeFind command - broken in recent refactoring
> Updating creator.vim file (this fixes #352)
> continue breaking down the epic autoload module
> continue breaking up the autoload/nerdtree god module
> move ui functions out into their own autoload
> add `.refreshFlags()` to path/dir/file objects
> fix a retarded bug with FlagSet.clearFlags
> allow flags to be scoped to a plugin
> add proof of concept for path flags API and add git modified flags
> Fixes README for opening vim with no files
> comment and tweak the <LeftRelease> fix in 24561ad
> simplify the <cr> mapping definition
> Fix <CR> not working if active node map is special char such as <Right>
> uncomment out some needed syntax matching code... fail
> trivial highlighting update for consistency
> fix highlighting for readonly files
> remove the NERDChristmasTree option
> simplify the syntax matching code slightly
> separate out syntax matching for Dir Arrows vs old style
> update symlink highlighting
> update the doc/quickhelp for :Bookmark
> update code in readme to auto-open a tree on startup
> update the NERDTreeCascadeOpenSingleChildDir doc
> rename NERDTreeWildIgnore option and refactor the usage of it
> Option for mixing dot files when sorting
> Support space in path when renaming
> Add flag to disable/enable bookmark sorting
> Use pathogen.vim Helptags command in README file
> Issue 315 - Incorrect buffer deleted on file delete
> correct invalid reference to MenuController
> Add option to respect wildignore
> Fix typo
> Ugly hack to fix Error 121 when NERDTree is the active window and clicking on the command line below another window.
> Create nested parent directories as needed
> Fix help mistake
> Fixed typo in NERD_tree.txt
> fix spelling in documentation
> Revert "don't redefine user-defined mappings"
> don't redefine user-defined mappings
> Add correspoding close action to cascade open single child dir
> Fix typo Casade to Cascade
> move the class files out of the plugin dir
> fix a bug where the user couldnt override the mapping for <cr>
> fix NERDTreeCWD
> Set filetype unconditionally and after defining mappings.
> catch an error thrown when refreshing a dir with a FIFO or socket
> move a couple of functions into NERDTreeCreator
> add some class methods to wrap the NERDTreeCreator public methods
> fixed error messages on copy node in NERDTree buffer
> extract the tree creation functions out into their own class
> fix a couple of function calls that should have been renamed
> some cosmetic updates to the class files
> fix the <enter> mapping
> move KeyMap_Invoke into the autoload file
> manually apply #220 to fix NERDTreeFind for hidden dirs
> replace a script level var with a query
> decompose the giant NERD_tree.vim file
> Added a frequently asked question to README
> Fix s:Path.Resolve to it handles '/' properly
> add Path.isUnixHiddenFile and refactor to use it
> Add a NERDTreeCWD command to change tree root to CWD
> Make NERDTreeFind use an existing NERDTree window
> Make NERDTreeFind work on hidden files
> Fix call to globpath() for Vim 7.2 and below.
> Fix typo in copy action
> Better handling file renaming and deleting
> Better flow for renaming and deleting files
> update the doc for NERDTreeCasadeOpenSingleChildDir
> default NERDTreeCasadeOpenSingleChildDir to 1
> add option NERDTreeCasadeOpenSingleChildDir and function TreeDirNode.openAlong for Issues #179.
> Fixed typo. changing s:clearBoomarks to s:clearBookmarks.
> ignore wildignore setting
> add autocmds for fugitive integration
> Replace virtcol() with len() in handleLeftClick().
> add NERDTreeFocus command - calls the NERDTreeFocus() function
> fix previewing of directory nodes
> add NERDTreeFocus function
> Don't output a message on opening a file node
> Remove second "edit" from Opener
> Make handling of '<>' notation more robust.
> fix the ctrl-j/ctrl-k mappings
> Escape mappings that use '<>' notation.
> Add calls to NERDTreeAddKeyMap for previewing Bookmarks
> map <cr> to the default o mapping (again)
> dont close the tree window when using the 'preview' maps
> fix NERDTreeQuitOnOpen option
> Add s:Path.Resolve function to address trailing slash issues.
> Revert "Fixed resolve() double endslash defect"
> Deactivating folds in NERDtree buffer
> remove a now misleading comment
> make the mouse work as expected with NERDTreeMouseMode
> refactor KeyMap so we dont use index() on an array of objects
> make the default keymap generation happen only once
> unlet TreeDirNode.openInNewTab since we are overriding it
> update the method comment for TreeDirNode.open
> fix a bug with the T (open in new tab silently) mapping
> rename 'split' to 'where' in the open() interface
> add Opener class to handle opening nodes/bookmarks
> make Bookmark use the new open() interface
> Added comment to clarify which part of the path is the 'drive' for windows network shares
> wrap a couple of wincmd calls in s:exec
> trivial comment change
> remove s:openEntrySplit as it is not needed anymore
> rename 'preview' to 'stay' and standardize the 'keepopen' name
> make TreeDirNode use the new open() interface
> fix Bookmark so it uses the new TreeFileNode.open method
> refactor the TreeFileNode*open* methods
> add dir and file flags to NERDTreeIgnore regexes
> dont convert windows paths to lower case when :editing
> fix a bug for files that begin with +/-/~ chars
> update faq in the readme
> add NERDTreeCustomIgnoreFilter hook
> fix a bug when activating bookmarks
> dont use VimEnter to cache bookmarks and load plugins
> fix a bug where keymaps were failing with :set ignorecase
> make a syntax fix for certain versions of vim
> use the keymap api for the mouse mappings
> make a small fix for the left mouse button
> refactor option checking to simplify and remove duplication
> fix the mouse mappings
> update quickhelp for custom mappings
> update the doc for the new key api functionality
> remove a trivial piece of old debugging code
> make the standard mappings use the key api
> rework the keymap API
> stayInCurrentTab fix
> Fixed resolve() double endslash defect
> Added support for windows network shares (\\box\share format paths)