REPL history navigation should not mutate history by default

[adienes]

example pasted from slack:

julia> a = "PREVIOUS"
"PREVIOUS"

julia> a = 1
1

julia> <UP><DEL><DEL><UP><CTRL-C>

julia> <UP>

# navigates to `a =` but should be `a = 1`

description of mechanics from @rfourquet

Actually, what is mutated doesn't seem to be very predictable, it looks like the mutated buffer will generally be the new REPL entry, and the history itself is not mutated, except when in the mean time you navigated somewhere else in history

proposed change:

the in-memory representation of Julia repl_history.jl should not be mutated during any normal navigation, except when such mutation is explicitly opted into via some new keychord