Fix when match.translate_slashes is "off".

clink.filematches() always converted forward slashes to backslashes,
related to #57.  But it shouldn't convert slashes there, and also it
wasn't correct to treat `../foo` as a relative path anyway, since CMD
actually interprets `foo/bar` as `foo /bar` -- the slash is interpreted
as a switch character, not as a path separator.

Additionally, this change also makes "off" mode preserve typed forward
slashes and backslashes (e.g. completing `/foo/` became `/foo\bar\`,
but the expected behavior is really `/foo/bar\`).

clink/lua/scripts/arguments.lua

@@ -2220,7 +2220,11 @@ local function file_matches_impl(match_word, exact)
         return matches
     end
 
-    local root = (path.getdirectory(word) or ""):gsub("/", "\\")
+    local root = (path.getdirectory(word) or "")
+    if word:find("^/", #root + 1) == #root + 1 then
+        -- Preserve forward slash after the directory part, when present.
+        root = root.."/"
+    end
     if expanded then
         root = rl.collapsetilde(root)
     end