copies-rust: get the parents' copies earlier

This remove some conditional nesting and prepare for future work were we will
unify the handling of copies from p1 and p2.

Differential Revision: https://phab.mercurial-scm.org/D9646

rust/hg-core/src/copy_tracing.rs

@@ -397,14 +397,24 pub fn combine_changeset_copies<D>(
         // the individual copies information the curent revision.  Creating a
         // new TimeStampedPath for each `rev` → `children` vertex.
         let mut copies: Option<InternalPathCopies> = None;
-        if p1 != NULL_REVISION {
             // Retrieve data computed in a previous iteration
-            let parent_copies = get_and_clean_parent_copies(
+        let p1_copies = match p1 {
+            NULL_REVISION => None,
+            _ => get_and_clean_parent_copies(
                 &mut all_copies,
                 &mut children_count,
                 p1,
-            );
-            if let Some(parent_copies) = parent_copies {
+            ), // will be None if the vertex is not to be traversed
+        };
+        let p2_copies = match p2 {
+            NULL_REVISION => None,
+            _ => get_and_clean_parent_copies(
+                &mut all_copies,
+                &mut children_count,
+                p2,
+            ), // will be None if the vertex is not to be traversed
+        };
+        if let Some(parent_copies) = p1_copies {
                 // combine it with data for that revision
                 let vertex_copies = add_from_changes(
                     &mut path_map,
@@ -416,15 +426,7 pub fn combine_changeset_copies<D>(
                 // keep that data around for potential later combination
                 copies = Some(vertex_copies);
             }
-        }
-        if p2 != NULL_REVISION {
-            // Retrieve data computed in a previous iteration
-            let parent_copies = get_and_clean_parent_copies(
-                &mut all_copies,
-                &mut children_count,
-                p2,
-            );
-            if let Some(parent_copies) = parent_copies {
+        if let Some(parent_copies) = p2_copies {
                 // combine it with data for that revision
                 let vertex_copies = add_from_changes(
                     &mut path_map,
@@ -450,7 +452,6 pub fn combine_changeset_copies<D>(
                     )),
                 };
             }
-        }
         match copies {
             Some(copies) => {
                 all_copies.insert(rev, copies);