Introduce `history_since_<id>` value expansion

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

    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}
    }

1 files changed, 5 insertions, 0 deletions

diff --git a/doc/pages/expansions.asciidoc b/doc/pages/expansions.asciidoc
index cf55902f..37e5811c 100644
--- a/doc/pages/expansions.asciidoc
+++ b/doc/pages/expansions.asciidoc
@@ -305,6 +305,11 @@ The following expansions are supported (with required context _in italics_):
     history), and each _modification_ is presented as in
     `%val{uncommitted_modifications}`.
 
+*%val{history_since_id}*::
+    _in buffer, window scope_ +
+    a partial history of the buffer in the same format as `%val{history}`
+    starting after entry _id_
+
 *%val{history_id}*::
     _in buffer, window scope_ +
     history id of the current buffer, an integer value which refers to a