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.
created new head
digraph G {
-1 -> 0
0 -> 1
0 -> 2
2 -> 3
1 -> 3
}