rewriteutil: add a precheck function to validate rewrite beforehand
For now this only checks for changeset mutability and this is only used by
'checkfold'. We'll expand the checking and usage as we go.
#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