Limit ctrl-r autofill/replace to a single command substitution

As reported on gitter, commands like "rm (...)" sometimes want a previous
command inside the parentheses. Let's try that.  If a user actually wants
to search for a command substitution they can move the cursor outside the
command substitution, or type the search string after pressing ctrl-r?

CHANGELOG.rst

@@ -62,7 +62,7 @@ Notable improvements and fixes
 - :kbd:`ctrl-c` during command input no longer prints ``^C`` and a new prompt but merely clears the command line. This restores the behavior from version 2.2. To revert to the old behavior use ``bind ctrl-c __fish_cancel_commandline`` (:issue:`10213`).
 - Undo history is no longer truncated after every command but kept for the lifetime of the shell process.
 - The :kbd:`ctrl-r` history search now uses glob syntax (:issue:`10131`).
-- The :kbd:`ctrl-r` history search now operates only on the line at cursor, making it easier to quickly compose a multi-line command by recalling previous commands.
+- The :kbd:`ctrl-r` history search now operates only on the line or command substitution at cursor, making it easier to combine commands from history.
 - Abbreviations can now be restricted to specific commands. For instance::
 
     abbr --add --command git back 'reset --hard HEAD^'

src/reader.rs

@@ -2570,7 +2570,17 @@ impl ReaderData {
                 let search_string = if !self.history_search.active()
                     || self.history_search.search_string().is_empty()
                 {
-                    parse_util_escape_wildcards(self.command_line.line_at_cursor())
+                    let cmdsub = parse_util_cmdsubst_extent(
+                        self.command_line.text(),
+                        self.command_line.position(),
+                    );
+                    let cmdsub = &self.command_line.text()[cmdsub];
+                    let needle = if !cmdsub.contains('\n') {
+                        cmdsub
+                    } else {
+                        self.command_line.line_at_cursor()
+                    };
+                    parse_util_escape_wildcards(needle)
                 } 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.
@@ -5180,7 +5190,13 @@ pub fn completion_apply_to_command_line(
 
     if do_replace_line {
         assert!(!do_escape, "unsupported completion flag");
-        return replace_line_at_cursor(command_line, inout_cursor_pos, val_str);
+        let cmdsub = parse_util_cmdsubst_extent(command_line, cursor_pos);
+        return if !command_line[cmdsub.clone()].contains('\n') {
+            *inout_cursor_pos = cmdsub.start + val_str.len();
+            command_line[..cmdsub.start].to_owned() + val_str + &command_line[cmdsub.end..]
+        } else {
+            replace_line_at_cursor(command_line, inout_cursor_pos, val_str)
+        };
     }
 
     let mut escape_flags = EscapeFlags::empty();