repeat#set() so far automatically incremented b:changedtick. Problems
with this:
1. The way that was done clobbered the expression register "=.
2. It causes the "readonly" warning and "Cannot make changes" error in
readonly/nomodifiable buffers, so mappings that don't modify anything
cannot be repeated there.
3. It's actually not needed most of the time, because many user mappings
and all repeatable Vim built-in normal mode commands I know (with the
exception of yank with cpo+=y) actually do modify the buffer
themselves.
For the exceptional case where the user has a set of related mappings,
one that repeats naturally (e.g. a custom operator, via g@), and one
that invokes repeat#set(), and both do not modify the buffer, a new
function repeat#invalidate() is offered. This should be called by the
former mapping, and all is well.
476c28084e