Do not update the unders if they are user defined

Closes jupyter/notebook#1628 (once there are tests).

Still shift the internal reference to self._,__,___ but do not assign it
in user ns.

I would expect this to be a change of behavior and have the (slight)
side effect that if any of the _, __, or __ are defined, then none of
these are updated.

We could update the logic to do the right think on leapfrog _, __ or __
if user-set, but is it worth it.

If _ is in builtins, then _ will not update the history and _ will be
the builtins, unless user set it explicitly in which case it takes
precedence.

Summary
    if user have set _
       _ == value set by the user
    elif _ in builtins:
        _ == value in builtins._
    else:
        _ == previous result.

Note that this logic may fall down if the use set _ to a specific value,
and have this save value returned while _ is also in builtin:

    In [1]: import gettext ; gettext.install('foo')

    In [2]: _ = 'Example'

    In [3]: _
    Out[3]: 'Example'

    In [4]: _
    Out[4]: 'Example'

    In [5]: _
    Out[5]: <bound method NullTranslations.gettext of <...>>

IPython/core/displayhook.py

@@ -76,6 +76,9 b' class DisplayHook(Configurable):'
         # particular uses _, so we need to stay away from it.
         if '_' in builtin_mod.__dict__:
             try:
+                user_value = self.shell.user_ns['_']
+                if user_value is not self._:
+                    return
                 del self.shell.user_ns['_']
             except KeyError:
                 pass
@@ -195,13 +198,23 b' class DisplayHook(Configurable):'
         if result is not self.shell.user_ns['_oh']:
             if len(self.shell.user_ns['_oh']) >= self.cache_size and self.do_full_cache:
                 self.cull_cache()
-            # Don't overwrite '_' and friends if '_' is in __builtin__ (otherwise
-            # we cause buggy behavior for things like gettext).
 
-            if '_' not in builtin_mod.__dict__:
-                self.___ = self.__
-                self.__ = self._
-                self._ = result
+            # Don't overwrite '_' and friends if '_' is in __builtin__
+            # (otherwise we cause buggy behavior for things like gettext). and
+            # do not overwrite _, __ or ___ if one of these has been assigned
+            # by the user.
+            update_unders = True
+            for unders in ['_'*i for i in range(1,4)]:
+                if not unders in self.shell.user_ns:
+                    continue
+                if getattr(self, unders) is not self.shell.user_ns.get(unders):
+                    update_unders = False
+
+            self.___ = self.__
+            self.__ = self._
+            self._ = result
+
+            if ('_' not in builtin_mod.__dict__) and (update_unders):
                 self.shell.push({'_':self._,
                                  '__':self.__,
                                 '___':self.___}, interactive=False)
@@ -209,7 +222,7 b' class DisplayHook(Configurable):'
             # hackish access to top-level  namespace to create _1,_2... dynamically
             to_main = {}
             if self.do_full_cache:
-                new_result = '_'+repr(self.prompt_count)
+                new_result = '_%s' % self.prompt_count
                 to_main[new_result] = result
                 self.shell.push(to_main, interactive=False)
                 self.shell.user_ns['_oh'][self.prompt_count] = result

IPython/core/tests/test_displayhook.py

@@ -26,3 +26,30 b' def test_output_quiet():'
 
     with AssertNotPrints('2'):
         ip.run_cell('1+1;\n#commented_out_function()', store_history=True)
+
+def test_underscore_no_overrite_user():
+    ip.run_cell('_ = 42', store_history=True)
+    ip.run_cell('1+1', store_history=True)
+
+    with AssertPrints('42'):
+        ip.run_cell('print(_)', store_history=True)
+
+    ip.run_cell('del _', store_history=True)
+    ip.run_cell('6+6', store_history=True)
+    with AssertPrints('12'):
+        ip.run_cell('_', store_history=True)
+
+
+def test_underscore_no_overrite_builtins():
+    ip.run_cell("import gettext ; gettext.install('foo')", store_history=True)
+    ip.run_cell('3+3', store_history=True)
+
+    with AssertPrints('gettext'):
+        ip.run_cell('print(_)', store_history=True)
+
+    ip.run_cell('_ = "userset"', store_history=True)
+
+    with AssertPrints('userset'):
+        ip.run_cell('print(_)', store_history=True)
+    ip.run_cell('import builtins; del builtins._')
+