fix(scp): keep tilde in local paths by using _filedir instead of _expand

Do not replace `~` by `/home/user/` for local files to get behavior
consistent with other tools like `ls`. I hope, dropping `_expand` and
adding `_filedir` to have file name completion will not cause issues like
[Ville Skyttä \[Bash-completion-devel\] `_expand`, scp and quoting](https://alioth-lists-archive.debian.net/pipermail/bash-completion-devel/2009-February/000973.html)
when a patch intended to fix
[Debian #521406. bash-completion: leading tilde always expanded](https://bugs.debian.org/521406)
was reverted.

Author: maxnikulin

completions/ssh

@@ -557,8 +557,6 @@ _scp()
             ;;
     esac
 
-    _expand || return
-
     case $cur in
         !(*:*)/* | [.~]*) ;; # looks like a path
         *:*)
@@ -581,7 +579,8 @@ _scp()
                 return
                 ;;
             */* | [.~]*)
-                # not a known host, pass through
+                # not a known host
+                _filedir
                 ;;
             *)
                 _known_hosts_real ${ipvx-} -c -a \