Add more comments to explain logic in getNextSelectionRange

Co-Authored-By: Vanessa Yuen <vanessayuenn@users.noreply.github.com>

lib/models/patch/multi-file-patch.js

@@ -136,7 +136,7 @@ export default class MultiFilePatch {
 
     let lastSelectionIndex = 0;
     // counts unselected lines in changed regions from the old patch
-    // until we get to the bottom-most selected line from the old patch.
+    // until we get to the bottom-most selected line from the old patch (lastMax).
     patchLoop: for (const lastFilePatch of lastMultiFilePatch.getFilePatches()) {
       for (const hunk of lastFilePatch.getHunks()) {
         let includesMax = false;
@@ -160,13 +160,18 @@ export default class MultiFilePatch {
       }
     }
 
+    // Iterate over changed lines in new patch in order to find the
+    // new row to be selected based on the last selection index.
+    // As we walk through the changed lines, we whittle down the
+    // remaining lines until we reach the row that corresponds to the
+    // last selected index
+
     let newSelectionRow = 0;
     let remainingChangedLines = lastSelectionIndex;
 
     let foundRow = false;
     let lastChangedRow;
 
-    // counts the same number of lines but backwards in the new patch.
     patchLoop: for (const filePatch of this.getFilePatches()) {
       for (const hunk of filePatch.getHunks()) {
         for (const change of hunk.getChanges()) {
@@ -182,7 +187,10 @@ export default class MultiFilePatch {
       }
     }
 
-    if (!foundRow) { // the selected line was the last changed line
+    // If we never got to the last selected index, that means it is
+    // no longer present in the new patch (ie. we staged the last line of the file).
+    // In this case we want the next selected line to be the last changed row in the file
+    if (!foundRow) {
       newSelectionRow = lastChangedRow;
     }