Tweak fish_function_path loading

Change how the fish_function_path is modified to ensure there's no
duplicates even if the fish_function_path contained a path that matches
a repository's function path.

Force fish_function_path to be set globally to avoid issues if it was
originally set universally. This does not remove the universal variable,
but it does mean that once tacklebox is loaded, changes to the universal
variable won't affect the shell.

tacklebox.fish

@@ -81,19 +81,30 @@ function __tacklebox_prepend_path --no-scope-shadowing --description \
     end
 end
 
+function __tacklebox_append_path --no-scope-shadowing --description \
+        'Append the given path, if it exists, to the specified path list (defaults to PATH)'
+    set -l path "$argv[1]"
+    set -l list PATH
+    if set -q argv[2]
+        set list $argv[2]
+    end
+
+    if test -d $path
+        # If the path is already in the list, skip it.
+        if not contains -- $path $$list
+            set -- $list $$list $path
+        end
+    end
+end
+
 ###
 # Configuration
 ###
 
 # Standardize function path, to be restored later.
-# Reset to just the datadir functions. At the end, we'll put the user
-# functions back on front, and the sysconf functions on the end in front
-# of the datadir functions, as is normally expected.
 set -l user_function_path $fish_function_path
-__tacklebox_strip_word "$__fish_sysconfdir/functions" user_function_path
-set -l no_sysconf $status
-__tacklebox_strip_word "$__fish_datadir/functions" user_function_path
-set fish_function_path "$__fish_datadir/functions"
+# Ensure the function path is global, not universal
+set -g fish_function_path "$__fish_datadir/functions"
 
 # Add all functions
 for repository in $tacklebox_path[-1..1]
@@ -118,11 +129,14 @@ if test -n "$tacklebox_theme"
 end
 
 # Add back the user and sysconf functions as appropriate
-set user_function_path $user_function_path $fish_function_path
-__tacklebox_strip_word "$__fish_datadir/functions" user_function_path
-__tacklebox_strip_word "$__fish_sysconfdir/functions" user_function_path # just in case
-and set no_sysconf 0
-if test $no_sysconf -eq 0
+for path in $fish_function_path
+    # don't append either system path
+    if not contains -- "$path" "$__fish_sysconfdir/functions" "$__fish_datadir/functions"
+        __tacklebox_append_path $path user_function_path
+    end
+end
+if __tacklebox_strip_word "$__fish_sysconfdir/functions" user_function_path
     set user_function_path $user_function_path "$__fish_sysconfdir/functions"
 end
-set fish_function_path $user_function_path "$__fish_datadir/functions"
+__tacklebox_strip_word "$__fish_datadir/functions" user_function_path
+set -g fish_function_path $user_function_path "$__fish_datadir/functions"