transaction: reorder unlinking .hg/journal and .hg/journal.backupfiles
After this reordering, absence of '.hg/journal' just before starting
new transaction means also absence of '.hg/journal.backupfiles'.
In this case, all temporary files for preceding transaction should be
completely unlinked, and HG_PENDING doesn't cause unintentional
reading stalled temporary files in.
Otherwise, 'repo.transaction()' raises exception with "run 'hg
recover' to clean up transaction" hint.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..