copies-rust: hide most of the comparison details inside a closure

The function that compares values needs various supporting elements that are
the same for each call. We are about to both make change to these element and
change to call sites in our upcoming work. So abstracting most of the details
will help to avoid conflict while these works happen in parallel.

Differential Revision: https://phab.mercurial-scm.org/D9426

rust/hg-core/src/copy_tracing.rs

@@ -463,6 +463,15 b' fn merge_copies_dict<A: Fn(Revision, Rev'
     changes: &ChangedFiles,
     oracle: &mut AncestorOracle<A>,
 ) -> TimeStampedPathCopies {
+    // This closure exist as temporary help while multiple developper are
+    // actively working on this code. Feel free to re-inline it once this
+    // code is more settled.
+    let mut cmp_value =
+        |dest: &HgPathBuf,
+         src_minor: &TimeStampedPathCopy,
+         src_major: &TimeStampedPathCopy| {
+            compare_value(changes, oracle, dest, src_minor, src_major)
+        };
     if minor.is_empty() {
         major
     } else if major.is_empty() {
@@ -499,9 +508,7 b' fn merge_copies_dict<A: Fn(Revision, Rev'
                 DiffItem::Update { old, new } => {
                     let (dest, src_major) = new;
                     let (_, src_minor) = old;
-                    match compare_value(
-                        changes, oracle, dest, src_minor, src_major,
-                    ) {
+                    match cmp_value(dest, src_minor, src_major) {
                         MergePick::Major => to_minor(dest, src_major),
                         MergePick::Minor => to_major(dest, src_minor),
                         // If the two entry are identical, no need to do