Map undo selection change to <a-u>/<a-U>

Change the initial <c-h>/<c-k> bindings to the recently freed-up
<a-u></a-U>.

Pros:
- easier to remember
- the redo binding is logical.
- works on legacy terminals, unlike <c-h>

Cons:
- It's less convenient to toggle between selection undo and redo
  keys. I think this is okay since this scenario does not happen that
  often in practice.

doc/pages/keys.asciidoc

@@ -308,10 +308,10 @@ Yanking (copying) and pasting use the *"* register by default (See <<registers#,
 *U*::
     redo last change
 
-*<c-h>*::
+*<a-u>*::
     undo last selection change
 
-*<c-k>*::
+*<a-U>*::
     redo last selection change
 
 *&*::

src/normal.cc

@@ -2340,8 +2340,8 @@ static constexpr HashMap<Key, NormalCmd, MemoryDomain::Undefined, KeymapBackend>
     { {'u'}, {"undo", undo} },
     { {'U'}, {"redo", redo} },
 
-    { {ctrl('h')}, {"undo selection change", undo_selection_change<Backward>} },
+    { {alt('u')}, {"undo selection change", undo_selection_change<Backward>} },
-    { {ctrl('k')}, {"redo selection change", undo_selection_change<Forward>} },
+    { {alt('U')}, {"redo selection change", undo_selection_change<Forward>} },
 
     { {alt('i')}, {"select inner object", select_object<ObjectFlags::ToBegin | ObjectFlags::ToEnd | ObjectFlags::Inner>} },
     { {alt('a')}, {"select whole object", select_object<ObjectFlags::ToBegin | ObjectFlags::ToEnd>} },

test/normal/selection-undo/fold-redundant-entries/script

@@ -1,2 +1,2 @@
 ui_out -ignore 4
-ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "gjgkxd<c-h>ihere<esc>" ] }'
+ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "gjgkxd<a-u>ihere<esc>" ] }'

test/normal/selection-undo/redo/script

@@ -1,2 +1,2 @@
 ui_out -ignore 4
-ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "2jj<c-h><c-h><c-k>ihere<esc>" ] }'
+ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "2jj<a-u><a-u><a-U>ihere<esc>" ] }'

test/normal/selection-undo/undo/script

@@ -1,2 +1,2 @@
 ui_out -ignore 4
-ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "j2j<c-h><c-h>ihere<esc>" ] }'
+ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "j2j<a-u><a-u>ihere<esc>" ] }'

test/normal/selection-undo/windisplay-hook/script

@@ -1,2 +1,2 @@
 ui_out -ignore 4
-ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "j:buffer *debug*<ret><c-h>ihere<esc>" ] }'
+ui_in '{ "jsonrpc": "2.0", "method": "keys", "params": [ "j:buffer *debug*<ret><a-u>ihere<esc>" ] }'