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}
    }

Author: pjungkamp

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

src/buffer_utils.cc

@@ -392,7 +392,7 @@ static String modification_as_string(const Buffer::Modification& modification)
                   modification.content->strview());
 }
 
-Vector<String> history_as_strings(const Vector<Buffer::HistoryNode>& history)
+Vector<String> history_as_strings(ConstArrayView<Buffer::HistoryNode> history)
 {
     Vector<String> res;
     for (auto& node : history)

src/buffer_utils.hh

@@ -97,7 +97,7 @@ void write_buffer_to_backup_file(Buffer& buffer);
 
 void write_to_debug_buffer(StringView str);
 
-Vector<String> history_as_strings(const Vector<Buffer::HistoryNode>& history);
+Vector<String> history_as_strings(ConstArrayView<Buffer::HistoryNode> history);
 Vector<String> undo_group_as_strings(const Buffer::UndoGroup& undo_group);
 
 String generate_buffer_name(StringView pattern);

src/main.cc

@@ -412,6 +412,13 @@ static const EnvVarDesc builtin_env_vars[] = { {
         "history", false,
         [](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>