cmLocalGenerator: Factor out relative path conversion helpers

Most calls to `MaybeConvertToRelativePath` use one of our common work
directories (e.g. top of the build tree) as the local path.  Add helpers
for each of the common cases to simplify and clarify call sites.

Source/cmLocalVisualStudioGenerator.cxx

@@ -207,10 +207,8 @@ std::string cmLocalVisualStudioGenerator::ConstructScript(
     }
 
     if (workingDirectory.empty()) {
-      script +=
-        this->ConvertToOutputFormat(this->MaybeConvertToRelativePath(
-                                      this->GetCurrentBinaryDirectory(), cmd),
-                                    cmOutputConverter::SHELL);
+      script += this->ConvertToOutputFormat(
+        this->MaybeRelativeToCurBinDir(cmd), cmOutputConverter::SHELL);
     } else {
       script += this->ConvertToOutputFormat(cmd.c_str(), SHELL);
     }