Check for tags in the proper git repository
Fix missing functions
Reuse installed git for tag builds

Reinstalling it over and over again messes up Windows' ability
to uninstall the other versions
Some whitespace cleanup

Includes removal of commented code
Make the tag build preparation a little more verbose
Buildserver: set GNUCASH_SCM_REV when building a tag

The buildscripts use this variable to make build decisions.
Refine check whether to build boost or not