* Fix Bug #7054: Reuse of $:/core/ui/EditTemplate/fields broken
This fixes Bug #7054 by creating a new variable
`safeNewFieldValueTiddlerPrefix` that is always set to a valid,
temporary prefix. This variable is used within `$action-deletetiddler`.
If the passed `newFieldValueTiddlerPrefix` variable is empty a new
prefix will be generated by the same logic that creates the original
`newFieldValueTiddlerPrefix` variable.
To be even more defensive, the prefix-filter was prepended with another
prefix filter that limits matches to `$:/temp/NewFieldValue`. This
prevents a bogus but non empty value in `newFieldValueTiddlerPrefix`
to delete arbitrary tiddlers.
* Add a default for `newFieldValueTiddlerPrefix`
This fixes a problem that was identified while fixing bug #7054. If the
tiddler `$:/core/ui/EditTemplate/fields` is transcluded directly and
`newFieldValueTiddlerPrefix` is not set, it will be generated. This
makes sure that this value is defined. It can not be redefined
unconditionally because if this tiddler is transcluded from
`EditTemplate.tid` these two tiddlers must agree on the same prefix
because the clean-up is duplicated between `EditTemplate.tid` and
`fields.tid`.
This would make `safeNewFieldValueTiddlerPrefix` obsolete, but I leave
it in there to make the macros safe and prevent any problems if the
`newFieldValueTiddlerPrefix` is unset by a later change.
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