List all changes related to a build.

 - to do this, the column 'parent_changeid' was added to the table changes

Note:
 - I added a tab 'changes' for the build webpage. I think this should be modified in order to have something more readable (but my angularJS skills are very limited).
Hopefully getLastChangeid() is a good method to find out the parent.

- a merge commit as several parents
- what if somebody force push a branch

At least we should document that at the moment we support only the trivial case.

I think eventually parent information should come from the change source