Optimize prompt performance

- Replace external text-processing pipelines with fish builtins and loops.
 - Use cheaper prompt data sources for git state, user names, and path basenames.
 - Add native dotted-version comparison for Go prompt checks.

functions/fish_title.fish

@@ -3,9 +3,11 @@
 function __bobthefish_title_user -S -d 'Display actual user if different from $default_user'
     if [ "$theme_title_display_user" = yes ]
         if [ "$USER" != "$default_user" -o -n "$SSH_CLIENT" ]
-            set -l IFS .
-            uname -n | read -l host __
-            echo -ns (whoami) '@' $host ' '
+            set -l host (string split . -- (uname -n))[1]
+            [ -n "$USER" ]
+            and echo -ns $USER
+            or echo -ns (whoami)
+            echo -ns '@' $host ' '
         end
     end
 end