hg-core: silence dead-code warning by adding RevlogEntry::revion() accessor

Nightly `rustc` warns about the `RevlogEntry::rev` field not being
used. Rather than removing it, I added an accessor since it seems
useful to be able to get the entry's revision.

Differential Revision: https://phab.mercurial-scm.org/D11548

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

@@ -304,6 +304,10 b" pub struct RevlogEntry<'a> {"
 }
 
 impl<'a> RevlogEntry<'a> {
+    pub fn revision(&self) -> Revision {
+        self.rev
+    }
+
     /// Extract the data contained in the entry.
     pub fn data(&self) -> Result<Cow<'_, [u8]>, RevlogError> {
         if self.bytes.is_empty() {