[main-] fix undo of first user alteration to a replayed cmdlog

[midichef]

When I replay a command log, like with echo |vd -p - then modify the sheet i, and try to undo it with U, I get a fail error: nothing to undo on current sheet.

That's because undo skips the first command in the log (after skipping any earlier options-setting commands), expecting it to be the command that created the sheet:

visidata/visidata/undo.py

Lines 44 to 46 in 641a5ea

	# skip the first remaining command, to exclude it from undo,
	# because it is always the one that created the sheet
	for i, cmdlogrow in enumerate(reversed(list(cmdlogrows)[1:])):

That's true for most sheets, but the replayed cmdlog sheet does not have a sheet creation command, and it breaks the expectation.

This PR fixes the undo by inserting a no-op as the first command.