Fix new NERDTrees' width when previous one was in the only window.

[PhilRunninger]

When leaving a NERDTree buffer, its window's width is remembered so that
that width can be used when showing the buffer again in a new window. If
NERDTree is the only window when leaving the buffer, it remembers the
whole width of the editor. When a new NERDTree window is created, it is
sized such that there is only a very small window for files to be opened
into.

To fix this, if NERDTree is the ONLY window, remember its width as the
value of g:NERDTreeWinSize, not the width of the editor.

Description of Changes

Closes #1152

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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

[leighmcculloch]

This fixes the problem! Thanks!