rhg: A missing .hg/dirstate file is not an error

Instead treat it as like an empty file

Differential Revision: https://phab.mercurial-scm.org/D10766

rust/rhg/src/commands/status.rs

@@ -9,6 +9,7 b' use crate::error::CommandError;'
 use crate::ui::Ui;
 use clap::{Arg, SubCommand};
 use hg;
+use hg::errors::HgResultExt;
 use hg::errors::IoResultExt;
 use hg::matchers::AlwaysMatcher;
 use hg::operations::cat;
@@ -164,8 +165,13 b' pub fn run(invocation: &crate::CliInvoca'
 
     let repo = invocation.repo?;
     let mut dmap = DirstateMap::new();
-    let dirstate_data = repo.hg_vfs().mmap_open("dirstate")?;
-    let parents = dmap.read(&dirstate_data)?;
+    let dirstate_data =
+        repo.hg_vfs().mmap_open("dirstate").io_not_found_as_none()?;
+    let dirstate_data = match &dirstate_data {
+        Some(mmap) => &**mmap,
+        None => b"",
+    };
+    let parents = dmap.read(dirstate_data)?;
     let options = StatusOptions {
         // TODO should be provided by the dirstate parsing and
         // hence be stored on dmap. Using a value that assumes we aren't