copies-rust: rename Oracle.is_ancestor to Oracle.is_overwrite

The core information that we want here is about "does information from revision
X overwrite information in Y". To do so, we check is X is an ancestors of Y, but
this is an implementation details, they could be other ways. We update the
naming to clarify this (and align more with wording used in upcoming changesets.

For people curious about other ways: for example we could record the overwrite
graph as it happens and reuse that to check if X overwrite Y, without having to
do potential expensive `is_ancestor` call on the revision graph.

Differential Revision: https://phab.mercurial-scm.org/D9496

rust/hg-core/src/copy_tracing.rs

@@ -244,7 +244,7 b" impl<'a, A: Fn(Revision, Revision) -> bo"
     }
 
     /// returns `true` if `anc` is an ancestors of `desc`, `false` otherwise
-    fn is_ancestor(&mut self, anc: Revision, desc: Revision) -> bool {
+    fn is_overwrite(&mut self, anc: Revision, desc: Revision) -> bool {
         if anc > desc {
             false
         } else if anc == desc {
@@ -705,7 +705,7 b' fn compare_value<A: Fn(Revision, Revisio'
         if src_major.rev == src_minor.rev {
             // If the two entry are identical, they are both valid
             MergePick::Any
-        } else if oracle.is_ancestor(src_major.rev, src_minor.rev) {
+        } else if oracle.is_overwrite(src_major.rev, src_minor.rev) {
             MergePick::Minor
         } else {
             MergePick::Major
@@ -733,7 +733,7 b' fn compare_value<A: Fn(Revision, Revisio'
             // from each side might conflict.  The major side will
             // win such conflict.
             MergePick::Major
-        } else if oracle.is_ancestor(src_major.rev, src_minor.rev) {
+        } else if oracle.is_overwrite(src_major.rev, src_minor.rev) {
             // If the minor side is strictly newer than the major
             // side, it should be kept.
             MergePick::Minor
@@ -741,7 +741,7 b' fn compare_value<A: Fn(Revision, Revisio'
             // without any special case, the "major" value win
             // other the "minor" one.
             MergePick::Major
-        } else if oracle.is_ancestor(src_minor.rev, src_major.rev) {
+        } else if oracle.is_overwrite(src_minor.rev, src_major.rev) {
             // the "major" rev is a direct ancestors of "minor",
             // any different value should
             // overwrite