commit: try hard to reuse p1 manifest if nothing changed This is all for commit reproducibility on "hg convert". With this change, p1 manifest is reused if ctx.files() *to be committed* is empty, and if new manifest entry is identical to p1. This is important property for "hg convert" since memctx.files() built from a convert source may be either a) more narrowed thanks to a committed ctx.files() which provides more accurate status, or b) containing redundant files because of sloppy filtering on e.g. octopus merge.