tests/test-dispatch.py.out

obsolete: flip `obstore.successors` and `obsolete.precursors`

People were confused by the fact `obstore.precursors` contained marker allowing
to find "precursors" and vice-versa.

This changeset changes their meaning to:

- precursors[x] -> set(markers on precursors edges of x)
- successors[x] -> set(markers on successors edges of x)

Some documentation is added to clarify the situation.

running: init test1
result: None
running: add foo
result: 0
running: commit -m commit1 -d 2000-01-01 foo
result: None
running: commit -m commit2 -d 2000-01-02 foo
result: None
running: log -r 0
changeset:   0:0e4634943879
user:        test
date:        Sat Jan 01 00:00:00 2000 +0000
summary:     commit1

result: None
running: log -r tip
changeset:   1:45589e459b2e
tag:         tip
user:        test
date:        Sun Jan 02 00:00:00 2000 +0000
summary:     commit2

result: None