* Introduce a flag to to sort by the node's modification time.
This flag is part of the NERDTreeSortOrder list. '[[timestamp]]' sorts
oldest to newest, while '[[-timestamp]]' sorts newest to oldest. Its
position in the array determines whether it is the primary sort (first
position) or secondary sort (any other position). The remaining elements
in the array describe the other sort level following all prior rules. If
needed, a tertiary sort is done alphabetically on the filename.
Examples:
['[[timestamp]]'] - oldest to newest
['\/$','*','[[-timestamp]]'] - dirs, then files, each newest first.
When using this flag, performance is slowed a little because the sortKey
is always recalculated with every sort. Why? Because if a file is saved
(either in vim or by an outside program), NERDTree needs to go back out
to the shell to get the new modification time.
* Update NERDTreeSortOrder documentation.
The new `[[timestamp]]' and '[[-timestamp]]' flags are added to support
sorting by modification time.
* Fix the inequality. Must be >= to force recalculation of _sortKey.
* Move `let path=` closer to where it's used.
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