Add missing calls to the shellescape() function (#1099)

* Improve "g:NERDTreeQuickLook()"

The following improvements were made...

  - Use variable sigils
  - Shorten a local variable name
  - Prefer an early return over testing for a negative
  - Switch to single quotes
  - Call "shellescape()" to pass a command argument [IMPORTANT!]

The final change is a critical fix for the security and reliability
of this function (see ":h system()").

Similar fixes for the other functions in this script will follow.

* Improve "g:NERDTreeRevealInFinder()"

This commit makes several style improvements and adds a missing call
to the "shellescape()" function.

See also: 56cfbcff1e

* Improve "g:NERDTreeExecuteFile()"

Refer to: 56cfbcff1e

* Improve "g:NERDTreeRevealFileLinux()"

Refer to: 56cfbcff1e

* Improve "g:NERDTreeExecuteFileLinux()"

Refer to: 56cfbcff1e

* Properly reveal "/" on Linux

This commit handles the edge case where a user invokes the "reveal"
function on "/" on a Linux box.  There is nothing to do but open the
root directory itself since "/" has no parent.

* Update the "CHANGELOG.md" file

* Add final missing "shellescape()" calls

I initially thought that there were several more locations where
a call to "shellescape()" was required but omitted.  However, there
are only two.  I suppose I should have taken the time to look.

Fixing these was easy.  I would be surprised if this change breaks
anything on the user side.

* Update the "CHANGELOG.md" file (again)

Use a more fitting description of the change...

CHANGELOG.md

@@ -5,6 +5,7 @@
         - **.PATCH**: Pull Request Title (PR Author) [PR Number](Link to PR)
 -->
 #### 6.7
+- **.4**: Add missing calls to the `shellescape()` function (lifecrisis) [#1099](https://github.com/preservim/nerdtree/pull/1099)
 - **.3**: Fix vsplit to not open empty buffers when opening previously closed file (AwkwardKore) [#1098](https://github.com/preservim/nerdtree/pull/1098)
 - **.2**: Fix infinity loop (on winvim) in FindParentVCSRoot (Eugenij-W) [#1095](https://github.com/preservim/nerdtree/pull/1095)
 - **.1**: File Move: Escape existing directory name when looking for open files. (PhilRunninger) [#1094](https://github.com/preservim/nerdtree/pull/1094)