Do not consider the tree root to be "cascadable"

[lifecrisis]

Description of Changes

Do not consider the tree root to be "cascadable"

This PR fixes an un-handled error condition. To reproduce:

1. mkdir -p foo/bar/
2. cd foo/
3. vim
4. :NERDTree
5. Position cursor on bar/ in the NERDTree and hit x.

The result is a nasty error message because the handler for this command continues searching upward because it thinks the root isCascadable().

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New Version Info

Author's Instructions

• Derive a new MAJOR.MINOR.PATCH version number. Increment the:
  • MAJOR version when you make incompatible API changes
  • MINOR version when you add functionality in a backwards-compatible manner
  • PATCH version when you make backwards-compatible bug fixes
• Update CHANGELOG.md, following the established pattern.

Collaborator's Instructions

• Review CHANGELOG.md, suggesting a different version number if necessary.
• After merge, tag the merge commit, e.g. git tag -a 3.1.4 -m "v3.1.4" && git push origin --tags