Fix replacing completions spuriously quoting ~

Commit 29dc30711 (Insert some completions with quotes instead of
backslashes, 2024-04-13) wrongly copmletes

	$ cat ~/space

to

	$ cat '~/path with spaces'

Today completions can be either replacing or appending.  We never quote
(but backslash-escape) appending completions (unless they "append"
to an empty token).  We always quote replacing completions. The
assumption in this part of the code is that replacing completions
can be quoted without changing meaning.

This assumption is wrong for tildes.  For the backslash-escaping code
path, we take care of this edge case via a special DONT_ESCAPE_TILDES
flag. However that flag does not take effect when using quotes for
escaping. Fix that.

Unfortunately, e97a4fab7 (Escape : and = in file completions,
2024-04-19) introduced a (hopefully temporary) code clone in
escape_separators, which made added an extra step to debugging here.

tests/checks/tmux-complete2.fish

@@ -0,0 +1,13 @@
+#RUN: %fish %s
+#REQUIRES: command -v tmux
+#REQUIRES: uname -r | grep -qv Microsoft
+# cautiously disable because tmux-complete.fish is disabled
+#REQUIRES: test -z "$CI"
+
+isolated-tmux-start
+
+isolated-tmux send-keys 'touch ~/"path with spaces"' Enter C-l \
+    'cat ~/space' Tab
+tmux-sleep
+isolated-tmux capture-pane -p
+# CHECK: prompt 1> cat ~/path\ with\ spaces