In Rider, there are features that allow committing individual lines instead of a whole file. You can select in the commit dialog which lines to include/omit. Also I recommended taking a look at Changelists feature that allows separating certain changes from what would be committed next (I believe it works line by line basis).
I don't know Rider, but I assume this is a limitation in all of their IDEs. Yes, you can commit only certain changes within a file. But you can only do that based on whole chunks. For example, if you add two new lines at the end of an existing file, you cannot commit only one line but not the other. The same limitation applies to changelists (at least in IntelliJ IDEA and Webstorm).
In Rider, there are features that allow committing individual lines instead of a whole file. You can select in the commit dialog which lines to include/omit. Also I recommended taking a look at Changelists feature that allows separating certain changes from what would be committed next (I believe it works line by line basis).