Since the `redo` action never goes back to `index: 0`,
we have to mark the clean index as being the current
non-empty index for the undo/redo navigation to work properly.
The problem is that if we never `undo`, the stack index stays at zero,
which is the empty container waiting for commands. In that situation,
if we save the document, it registers the clean index as being 1
(the non-empty index) but because the stack index has never left zero,
the document was being reported as modified, being out of sync with
the window modified state.
Since we keep a stack of command combos, let's call entries on the
stack "Combo" instead of "UndoCommandsContainer".
And since it has a vector of commands, let's call it "commands"
instead of "m_undo_vector".
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
