copies-rust: move the mapping merging into a else clause

We are going to add more cases, to it is time to stop using early returns and to
move everything in a single if/elif/else block for clarity.

Differential Revision: https://phab.mercurial-scm.org/D9425

rust/hg-core/src/copy_tracing.rs

@@ -464,10 +464,10 b' fn merge_copies_dict<A: Fn(Revision, Rev'
     oracle: &mut AncestorOracle<A>,
 ) -> TimeStampedPathCopies {
     if minor.is_empty() {
-        return major;
+        major
     } else if major.is_empty() {
-        return minor;
-    }
+        minor
+    } else {
     let mut override_minor = Vec::new();
     let mut override_major = Vec::new();
 
@@ -478,9 +478,9 b' fn merge_copies_dict<A: Fn(Revision, Rev'
         override_minor.push((k.clone(), v.clone()))
     };
 
-    // The diff function leverage detection of the identical subpart if minor
-    // and major has some common ancestors. This make it very fast is most
-    // case.
+        // The diff function leverage detection of the identical subpart if
+        // minor and major has some common ancestors. This make it very
+        // fast is most case.
     //
     // In case where the two map are vastly different in size, the current
     // approach is still slowish because the iteration will iterate over
@@ -532,6 +532,7 b' fn merge_copies_dict<A: Fn(Revision, Rev'
     }
     result
 }
+}
 
 /// represent the side that should prevail when merging two
 /// TimeStampedPathCopies