reader: Use existing search string when opening the history pager

I sometimes find myself doing something like this:

- Look for a commandline that includes "echo" (as an example)
- Type echo, press up a few times
- I can't immediately find what I'm looking for
- Press ctrl-r to open up the history pager
- It uses the current commandline as the search string,
  so now I'm looking for "echo foobar"

This makes it so if the search string already is in use, that's what
the history-pager picks as the initial search string.

(cherry picked from commit 5b44c26a19)

src/reader.cpp

@@ -3757,7 +3757,13 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
             pager.set_search_field_shown(true);
             pager.set_prefix(MB_CUR_MAX > 1 ? L"► " : L"> ", false /* highlight */);
             // Update the search field, which triggers the actual history search.
-            insert_string(&pager.search_field_line, command_line.text());
+            if (!history_search.active() || history_search.search_string().empty()) {
+                insert_string(&pager.search_field_line, command_line.text());
+            } else {
+                // If we have an actual history search already going, reuse that term
+                // - this is if the user looks around a bit and decides to switch to the pager.
+                insert_string(&pager.search_field_line, history_search.search_string());
+            }
             break;
         }
         case rl::backward_char: {