abbr: Bail early when the abbr already exists

This speeds up the common case of `abbr -a` calls in config.fish by
about 90% - from 900ms to 90ms for 100 calls.

share/functions/abbr.fish

@@ -72,6 +72,9 @@ function abbr --description "Manage abbreviations"
 
 	switch $mode
 	case 'add'
+		# Bail out early if the exact abbr is already in
+		# This depends on the separator staying the same, but that's the common case (config.fish)
+		contains -- $mode_arg $fish_user_abbreviations; and return 0
 		set -l key
 		set -l value
 		__fish_abbr_parse_entry $mode_arg key value