Complete current commandline token with __fish_complete_directories if

Akin to __fish_complete_suffix, __fish_complete_directories now attempts
to complete the current commandline token if no token is explicitly
passed in as an argument.

share/functions/__fish_complete_directories.fish

@@ -1,12 +1,17 @@
 #
 # Find directories that complete $argv[1], output them as completions
-# with description $argv[2] if defined, otherwise use 'Directory'
+# with description $argv[2] if defined, otherwise use 'Directory'.
+# If no arguments are provided, attempts to complete current commandline token.
 #
 function __fish_complete_directories -d "Complete directory prefixes" --argument comp desc
     if not set -q desc[1]
         set desc (_ "Directory")
     end
 
+    if not set -q comp[1]
+        set comp (commandline -ct)
+    end
+
     set -l dirs $comp*/
     if set -q dirs[1]
         printf "%s\t$desc\n" $dirs