narrow: fix commits of empty files The problem is that when committing a new file with empty contents (or in general empty file with filelog p1 = -1), hg commit with narrow doesn't create a filelog revision at all, which causes failures in further commands. The problem seems to be that: - hg thinks that instead of creating a new filelog revision, it can use the filelog's p1 (the nullrev) - because it thinks the file contents is the same in that revision and in p1 - because `narrowfilelog.cmp(nullrev, b'')` is True (unlike with `filelog.cmp`) It's not clear to me which `cmp` behaves better. But I think it makes sense to change the commit code to not to "reuse" the null rev when adding an empty file with filelog p1 == filelog p2 == -1. This is consistent with never writing the null rev in the manifest, which `hg verify` claims is an invariant: ``` inside/c@4: manifest refers to unknown revision 000000000000 ``` Differential Revision: https://phab.mercurial-scm.org/D11400