branch closing: mark closed branches with a 'close' extra
Adds a --close-branch option to commit.
When --close-branch is present the commit will mark the changeset
with close=1 in the changeset extras field.
If a regular changeset is added on top of a closed head the branch
is no longer considered closed, and thus re-opened.
% test --time
% test --profile
% test --lsprof