[MERGE REVIEW] Numbered backups

[Aaron Bentley]

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This patch adds support for numbered backups.  This means that repeated
reverts will produce a series like:

foo~
foo~1~
foo~2~

etc.

Since this doesn't use the conflict-handling mechanism, you can have an
infinite number of backups, subject only to the limitations of your
filesystem.

The intent of these changes is to reduce frustration with our system,
but I hope we will introduce better a better 'undo' in the future, i.e.
one which supports 'redo'.

Aaron
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