object_id: add ChangeId::reverse_hex() for convenience

Borrowed from #4470.

cli/src/cli_util.rs

@@ -64,7 +64,6 @@ use jj_lib::git;
 use jj_lib::git_backend::GitBackend;
 use jj_lib::gitignore::GitIgnoreError;
 use jj_lib::gitignore::GitIgnoreFile;
-use jj_lib::hex_util::to_reverse_hex;
 use jj_lib::id_prefix::IdPrefixContext;
 use jj_lib::matchers::Matcher;
 use jj_lib::merge::MergedTreeValue;
@@ -2657,9 +2656,7 @@ pub fn short_commit_hash(commit_id: &CommitId) -> String {
 }
 
 pub fn short_change_hash(change_id: &ChangeId) -> String {
-    // TODO: We could avoid the unwrap() and make this more efficient by converting
+    change_id.reverse_hex()[0..12].to_string()
-    // straight from binary.
-    to_reverse_hex(&change_id.hex()[0..12]).unwrap()
 }
 
 pub fn short_operation_hash(operation_id: &OperationId) -> String {

cli/src/commit_templater.rs

@@ -31,7 +31,6 @@ use jj_lib::fileset;
 use jj_lib::fileset::FilesetDiagnostics;
 use jj_lib::fileset::FilesetExpression;
 use jj_lib::git;
-use jj_lib::hex_util::to_reverse_hex;
 use jj_lib::id_prefix::IdPrefixContext;
 use jj_lib::id_prefix::IdPrefixIndex;
 use jj_lib::matchers::Matcher;
@@ -1249,11 +1248,7 @@ impl CommitOrChangeId {
     pub fn hex(&self) -> String {
         match self {
             CommitOrChangeId::Commit(id) => id.hex(),
-            CommitOrChangeId::Change(id) => {
+            CommitOrChangeId::Change(id) => id.reverse_hex(),
-                // TODO: We can avoid the unwrap() and make this more efficient by converting
-                // straight from bytes.
-                to_reverse_hex(&id.hex()).unwrap()
-            }
         }
     }
 

lib/src/backend.rs

@@ -25,6 +25,7 @@ use futures::stream::BoxStream;
 use thiserror::Error;
 
 use crate::content_hash::ContentHash;
+use crate::hex_util;
 use crate::index::Index;
 use crate::merge::Merge;
 use crate::object_id::id_type;
@@ -50,6 +51,16 @@ id_type!(pub FileId);
 id_type!(pub SymlinkId);
 id_type!(pub ConflictId);
 
+impl ChangeId {
+    /// Returns the hex string representation of this ID, which uses `z-k`
+    /// "digits" instead of `0-9a-f`.
+    pub fn reverse_hex(&self) -> String {
+        // TODO: We can avoid the unwrap() and make this more efficient by
+        // converting straight from bytes.
+        hex_util::to_reverse_hex(&self.hex()).unwrap()
+    }
+}
+
 #[derive(ContentHash, Debug, PartialEq, Eq, Clone, Copy, PartialOrd, Ord)]
 pub struct MillisSinceEpoch(pub i64);
 

lib/tests/test_revset.rs

@@ -29,7 +29,6 @@ use jj_lib::git;
 use jj_lib::git_backend::GitBackend;
 use jj_lib::graph::GraphEdge;
 use jj_lib::graph::ReverseGraphIterator;
-use jj_lib::hex_util::to_reverse_hex;
 use jj_lib::id_prefix::IdPrefixContext;
 use jj_lib::object_id::ObjectId;
 use jj_lib::op_store::RefTarget;
@@ -403,7 +402,7 @@ fn test_resolve_symbol_in_different_disambiguation_context() {
             .with_id_prefix_context(&id_prefix_context);
 
     // Sanity check
-    let change_hex = &to_reverse_hex(&commit2.change_id().hex()).unwrap();
+    let change_hex = commit2.change_id().reverse_hex();
     assert_eq!(
         symbol_resolver
             .resolve_symbol(repo2.as_ref(), &change_hex[0..1])