topics: use a dict for successors of changesets produced by topic change
This patch adds a dictionary to store the successors of changesets which got
obsoleted when we change it's topic. This is an improvement in the fix for
issue5441.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null