cmLocalGenerator: Localize logic mapping source path to object file name

We select an object file name based on the path to its source file.
Localize the logic for shortening this via relative paths.  It does not
need to use the generator-wide relative path conversion rules because we
are not actually generating a relative path that needs to be consistent
with anything else.

Source/cmLocalGenerator.cxx

@@ -3532,6 +3532,21 @@ bool cmLocalGenerator::IsNinjaMulti() const
   return this->GetState()->UseNinjaMulti();
 }
 
+namespace {
+std::string relativeIfUnder(std::string const& top, std::string const& cur,
+                            std::string const& path)
+{
+  // Use a path relative to 'cur' if it can be expressed without
+  // a `../` sequence that leaves 'top'.
+  if (cmSystemTools::IsSubDirectory(path, cur) ||
+      (cmSystemTools::IsSubDirectory(cur, top) &&
+       cmSystemTools::IsSubDirectory(path, top))) {
+    return cmSystemTools::ForceToRelativePath(cur, path);
+  }
+  return path;
+}
+}
+
 std::string cmLocalGenerator::GetObjectFileNameWithoutTarget(
   const cmSourceFile& source, std::string const& dir_max,
   bool* hasSourceExtension, char const* customOutputExtension)
@@ -3541,13 +3556,15 @@ std::string cmLocalGenerator::GetObjectFileNameWithoutTarget(
   std::string const& fullPath = source.GetFullPath();
 
   // Try referencing the source relative to the source tree.
-  std::string relFromSource = this->MaybeRelativeToCurSrcDir(fullPath);
+  std::string relFromSource = relativeIfUnder(
+    this->GetSourceDirectory(), this->GetCurrentSourceDirectory(), fullPath);
   assert(!relFromSource.empty());
   bool relSource = !cmSystemTools::FileIsFullPath(relFromSource);
   bool subSource = relSource && relFromSource[0] != '.';
 
   // Try referencing the source relative to the binary tree.
-  std::string relFromBinary = this->MaybeRelativeToCurBinDir(fullPath);
+  std::string relFromBinary = relativeIfUnder(
+    this->GetBinaryDirectory(), this->GetCurrentBinaryDirectory(), fullPath);
   assert(!relFromBinary.empty());
   bool relBinary = !cmSystemTools::FileIsFullPath(relFromBinary);
   bool subBinary = relBinary && relFromBinary[0] != '.';