import: allow processing of extra part header during import
As we have a way for extension to add more header, we need a way for them to
actually process them. We add a basic hook points to alter the changeset
(especially extra) before we commit. There would be more to do for a full
featured hooking, but this currently fit my needs.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.