merge: add tests for commit with no content change It isn't easy to say when to reuse the p1 manifest. Basically, that's only when wctx.files() is empty, but we need to know that wctx.files() is not the same as repo['.'].files() after the commit. This patch adds several examples of commits with empty ctx/wctx.files(). I don't think this is exhaustive, but it contains at least one failure mode in which a converted repo result in a different hash. I also note that the manifest revlog does NOT follow the DAG shape of the changelog since p1 manifest is reused if wctx.files() is empty even at merge. I don't know whether it is intentional or not, but it's the behavior since 2011, 301725c3df9a "localrepo: reuse parent manifest in commitctx if no files have changed."