tests: simplify by pruning unwanted changesets a little earlier
The pruned commits are not relevant to the test case, so let's clean
them up at the end of the previous test case instead of part-way
through the next one.
#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 -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null