Avoid rebuildAdd tree diffing the whole tree for revision.in HgLoaderFromDisk
Load time improvement on https://www.mercurial-scm.org/repo/hg/By looking at differences between revisions, the repository tree is
1:11:02 -> 47:58.84updated rather that fully rebuild for each one.
Observed load time improvement on https://www.mercurial-scm.org/repo/hg/
1:11:02 -> 47:58