treemanifest: speed up commit using dirty flag
We currently avoid saving a treemanifest revision if it's the same as
one of it's parents. This is checked by comparing the generated text
for all three versions. Let's avoid that when possible by comparing
the nodeids for clean (not dirty) nodes.
On the Mozilla repo, this speeds up commit from 2.836s to 2.343s.
Hello, Foobar
['Hello, Fo, =;o', 'Hello, Bar']