Introduce history_since_<id> value expansion

[pjungkamp]

Description

The history_since_<id> value expansion allows incremental parsing of a buffer's history.

Resolves #5345

Example

declare-option int my_last_history_id
define-command my-process-history ...

# process the initial buffer history
my-process-history %val{bufname} 0 %val{history}
set-option buffer my_last_history_id 0

# only process new history changes on idle
hook buffer NormalIdle %{
  evaluate-commands %exp{
    my-process-history \
      %%val{bufname} \
      %%opt{my_last_history_id} \
      %%val{history_since_%opt{my_last_history_id}}
  }
  set-option buffer my_last_history_id %val{history_id}
}

[mawww]

This looks big enough to warrant a copyright waiver, and I dont think I have one from you, can you add a copyright waiver commit to this PR (see the CONTRIBUTING file)

[pjungkamp]

You've already got one! (From my EnterDirectory hook PR)

07000ad

[mawww]

Indeed, sorry about that.