Remove unmaintained code
Pic macros from UMLGraph
Ensure that the repo exists before an entry is created
Use the head repo to register the commit with

Accidentaly, the base repo was used, which means
that all commits from pull_requests appeared
as direct commits to the main repo.
Permit commits to be inserted with null repositories

If the source of a commit cannot be identified (e.g.
because the repository has been deleted) add the
commit with no associated repository.
New 'merged' column in pull requests table

This enables pull requests to be marked as merged
by examining whether their commits also appear in
the base_repo_id commit list.
Fix incorrect assignments of pull request commits

Due to a bug (solved by 2bc9333b3a55), commits from
pull requests were registered to the base repo commit
list (instread of the head repo).
This script fixes the problem.