Currently, when attempting to merge a group of cells where more than one cell is non-empty, you'll get a dialog presenting 3 merge options every single time. We should add an option to turn this dialog off, and have Calc remember the ideal merge behavior and apply it without needing to launch the dialog.
Andras has already put the necessary piece in with commit da24355234ec12f2f4c652113dbbb1216ba5492c.