scmutil: add function to help handling workingctx in arithmetic operation
It's unfortunate that workingctx revision is None, which doesn't work well in
arithmetic operation or comparison. This function is trivial but will be used
in several places.
$ . "$TESTDIR/bzr-definitions"
$ cat > treeset.py <<EOF
> import sys
> from bzrlib import workingtree
> wt = workingtree.WorkingTree.open('.')
>
> message, rootid = sys.argv[1:]
> wt.set_root_id('tree_root-%s' % rootid)
> wt.commit(message)
> EOF
change the id of the tree root
$ mkdir test-change-treeroot-id
$ cd test-change-treeroot-id
$ bzr init -q source
$ cd source
$ echo content > file
$ bzr add -q file
$ bzr commit -q -m 'Initial add'
$ python ../../treeset.py 'Changed root' new
$ cd ..
$ hg convert source source-hg
initializing destination source-hg repository
scanning source...
sorting...
converting...
1 Initial add
0 Changed root
$ manifest source-hg tip
% manifest of tip
644 file
$ cd ..