Fix generates problems.

[trulede]

Fixes some problems with 'generates'.

1. Generates with timestamp method does not generate deleted files

Previously generated files, then deleted, are not regenerated when Task is subsequently run.

Task does not actually check the generates items exist, rather, it cunningly 'globs" each generates item and only notices those files which exist, and the timestamp file itself. That means, if you delete a file, task will not notice and because the timestamp file, created when the source last changed, was considered for the check ... another cunning move ... it will appear that the task is up-to-date.

Solution is to adopt the glob mechanism from checksum. If a generate item is globbed and returns no files, or a NotExist error, then return indicating that the Task should run.

2. Generates (and sources) items with a space in filename will be expanded correctly.

Curiously, this problem is often masked by the first problem. In any case, it seems that a fix from long ago has been lost. The fix is introduced again, at a slightly different location (i.e. the glob() function).

Original fix here.

fixes #2314
fixes #1506
fixes #1230

[trulede]

It might be worthwhile adding a few more test cases.