testdata: seed sandbox overlap child

Author: roodboi

_test_data/sandbox/overlap/shared.md

@@ -6,6 +6,6 @@ Parent branch owns this section and introduces the file.
 
 ## Section B
 
-This section will be edited by the child branch.
+Child branch rewrites this section to create same-file stacked diffs without a guaranteed merge conflict.
 
-Marker: 1639076
+Marker: b1537e2