commitctx: create the ChangingFiles object directly in the various case
No need to compute all data then create the object, we can create it early and
directly store data in it. We start simple by moving create higher in the
function, but the end goal is to eventually move the creation inside the
`_process_files` function to take advantage of the object there.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state