A conversion from Java to Kotlin in Android Studio 2.3.2 (in 3.0 the same behaviour) creates a new file and deletes previous. So Git doesn't know anything about this conversion. And the git history doesn't save. In Intellij Idea everything's fine. IDE just renames file and git saves the history. How can do the same in Android Studio.
Git guesses renames from added/removed file pairs, but only if these files are close enough, i.e. if the file was renamed with no or small amount of changes.
When you apply java-to-kotlin conversion usually every line of a file changes, so that git cannot find that these old and new files somehow relate to each other.
You can use the following two-stage approach instead:
.java file to .kt and commit it;.kt file.