purge_packages: Add vim modeline
purge_packages: Check for changes not merged upstream
purge_packages: Add option to switch on/off git gc
purge-packages: Allow CALL_GC to be determined by environment
Don't call git-gc by default in purge purge-packages.sh

Call to git-gc is time consuming and usually the benefits are not too
beneficial.
Merge changes in purge-packages.sh

* check for changes not merged upstream
* don't call git-gc by default
Add tool to reverse order parents in commit produced by pull

Merges produced by git pull from upstream repository results
in history with "wrong" first-parent ancestry. Tool fixmerge.sh
fixes such a commit. It also modifies the commit message to reflect
what was really merged where.