The undo stack was very difficult to understand as it grew by adding
new undo commands to the front of the internal vector. This meant we
had to keep updating indices as the stack grew and shrank.
This patch makes the internal vector grow by appending instead.
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 *
