diffstat: support filenames with whitespaces on renames This is a follow-up patch to D5628. `line.split()` cannot get filenames with whitespaces as mentioned by @yuja. This patch replaces `split()` method with `slice`. Corresponding tests were also added. Differential Revision: https://phab.mercurial-scm.org/D5709

largefiles: avoid walking full manifest When using treemanifest, diffs between two manifests can often be much cheaper than iterating the full manifests (because common subtrees are skipped). Differential Revision: https://phab.mercurial-scm.org/D5644

cleanup: use p1() instead of parents() when we only need the first parent Differential Revision: https://phab.mercurial-scm.org/D5708