cmdrewrite: use context manager for some locks and transactions
These were the trivial cases where I didn't even need to extend the
scope of a transaction or change indentation to fix it.
#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