reader: Only maintain cursor position in non-empty prefix search

Otherwise this would always move the cursor to the beginning.

Fixes #11133

(cherry picked from commit db244e0492)

src/reader.rs

@@ -1909,7 +1909,9 @@ fn update_command_line_from_history_search(&mut self) {
                     0..self.command_line.len(),
                     new_text,
                 );
-                if self.history_search.by_prefix() {
+                if self.history_search.by_prefix()
+                    && !self.history_search.search_string().is_empty()
+                {
                     self.command_line
                         .set_position(self.history_search.search_string().len());
                 }

tests/pexpects/bind.py

@@ -372,6 +372,14 @@ send('\x02\x02\x02') # ctrl-b, backward-char
 sendline('\x1bu') # alt+u, upcase word
 expect_prompt("fooBAR")
 
+sendline('bind ctrl-z history-prefix-search-backward')
+expect_prompt()
+sendline("echo this continues")
+expect_prompt()
+send("\x1A")
+sendline(" with this text")
+expect_prompt("this continues with this text")
+
 sendline("""
     bind ctrl-g "
         commandline --insert 'echo foo ar'