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, 7 insertions, 0 deletions

diff --git a/src/main.cc b/src/main.cc
index bf382dd2..fb78669d 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -413,6 +413,13 @@ static const EnvVarDesc builtin_env_vars[] = { {
         [](StringView name, const Context& context) -> Vector<String>
         { return history_as_strings(context.buffer().history()); }
     }, {
+        "history_since_", true,
+        [](StringView name, const Context& context) -> Vector<String>
+        { return history_as_strings(
+            ArrayView(context.buffer().history())
+                .subrange(str_to_int(name.substr(14_byte)) + 1)
+        ); }
+    }, {
         "uncommitted_modifications", false,
         [](StringView name, const Context& context) -> Vector<String>
         { return undo_group_as_strings(context.buffer().current_undo_group()); }