Update migration semantics to work better.

Update migration semantics to work better.

* .scripts-version is kept in its original place to
  make things more robust
* Backing up of .scripts and .git directories actually works
* Attempt to do all validation before doing any --force
  actions, to reduce broken migrations
* Remove .scripts/version generation
* More robust already migrated checks (check for .git and .scripts
  independently)
* Fix error in error handling code
* Turn on logging of effectful sh.eval() calls in upgrade
* Make parametrization more resistant against missing files
* Add log parameter, remove differing log messages based on eval
* Make pristine_tag and scripts_tag not braindead, fixing a bunch
  of bugs.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>