rhg: fix `hg cat` interaction with null revision

Differential Revision: https://phab.mercurial-scm.org/D11664

rust/hg-core/src/revlog/revlog.rs

@@ -133,6 +133,9 b' impl Revlog {'
     /// Returns the node ID for the given revision number, if it exists in this
     /// revlog
     pub fn node_from_rev(&self, rev: Revision) -> Option<&Node> {
+        if rev == NULL_REVISION {
+            return Some(&NULL_NODE);
+        }
         Some(self.index.get_entry(rev)?.hash())
     }
 

tests/test-rhg.t

@@ -129,8 +129,8 b' Specifying revisions by changeset ID'
   $ $NO_FALLBACK rhg cat -r d file-2
   2
   $ $NO_FALLBACK rhg cat -r 0000 file-2
-  abort: invalid revision identifier: 0000
-  [255]
+  file-2: no such file in rev 000000000000
+  [1]
 
 Cat files
   $ cd $TESTTMP