Don't clear cache when no database updates, fixes #755

[helmo]

No description provided.

[weitzman]

updb now has --cache-clear=0 if you want to manually control the cache step. we also have updatedb-status.

neither of those does what this PR does. someone needs to look at this closely. i probably wont do that myself given the available workarounds.

[helmo]

Both workarounds are just that ... :(

It just bugs me that caches are cleared when nothing has been done.
If I set --cache-clear=0 then I have to check myself if caches need to be flushed... and with the updatedb-status command I'm adding extra overhead and complexity.

[ergonlogic]

This looks pretty reasonable to me. It also matches Drupal's behaviour more closely. Drupal doesn't clear caches if there weren't any updates to apply.