Have Ctrl-C in search mode clear query, if present

As suggested in ipython/ipython#9314, when pressing Ctrl-C within the
search-bar, the current search terms will be cleared but the search
buffer will remain focused.  Pressing Ctrl-C when the search buffer is
empty will return focus to the default buffer.

IPython/terminal/ptshell.py

@@ -16,7 +16,7 b' from IPython.utils.process import abbrev_cwd'
 from traitlets import Bool, CBool, Unicode, Dict
 
 from prompt_toolkit.completion import Completer, Completion
-from prompt_toolkit.enums import DEFAULT_BUFFER
+from prompt_toolkit.enums import DEFAULT_BUFFER, SEARCH_BUFFER
 from prompt_toolkit.filters import HasFocus, HasSelection, Condition
 from prompt_toolkit.history import InMemoryHistory
 from prompt_toolkit.shortcuts import create_prompt_application, create_eventloop
@@ -182,6 +182,13 b' class TerminalInteractiveShell(InteractiveShell):'
         def _(event):
             event.current_buffer.reset()
 
+        @kbmanager.registry.add_binding(Keys.ControlC, filter=HasFocus(SEARCH_BUFFER))
+        def _(event):
+            if event.current_buffer.document.text:
+                event.current_buffer.reset()
+            else:
+                event.cli.push_focus(DEFAULT_BUFFER)
+
         supports_suspend = Condition(lambda cli: hasattr(signal, 'SIGTSTP'))
 
         @kbmanager.registry.add_binding(Keys.ControlZ, filter=supports_suspend)