rust-changelog: removed now useless early conditional for NULL_REVISION

Now that the underlying `RevlogEntry` returned for `NULL_REVISION` is
viable, it has become pointless to check NULL_REVISION several times,
even for performance: the check will be far more frequent than the benefit
of bailing earlier in the rare case where the requested revision is
`NULL_REVISION`.

This case is covered explicitly by the first assertion of
`test_data_from_rev_null`.

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

@@ -1,6 +1,6 b''
 use crate::errors::HgError;
+use crate::revlog::Revision;
 use crate::revlog::{Node, NodePrefix};
-use crate::revlog::{Revision, NULL_REVISION};
 use crate::revlog::{Revlog, RevlogEntry, RevlogError};
 use crate::utils::hg_path::HgPath;
 use crate::vfs::Vfs;
@@ -51,9 +51,6 b' impl Changelog {'
         &self,
         rev: Revision,
     ) -> Result<ChangelogRevisionData, RevlogError> {
-        if rev == NULL_REVISION {
-            return Ok(ChangelogRevisionData::null());
-        }
         self.entry_for_rev(rev)?.data()
     }