abbr.fish: escape the output of abbr --show

Allows abbreviations containing embedded newlines, etc., to be displayed
and exported properly.

Work on #731.

share/functions/abbr.fish

@@ -100,7 +100,9 @@ function abbr --description "Manage abbreviations"
 
 	case 'show'
 		for i in $fish_user_abbreviations
-			echo abbr -a \'$i\'
+			# Disable newline splitting
+			set -lx IFS ''
+			echo abbr -a \'(__fish_abbr_escape $i)\'
 		end
 		return 0
 
@@ -114,6 +116,10 @@ function abbr --description "Manage abbreviations"
 	end
 end
 
+function __fish_abbr_escape
+	echo $argv | sed -e s,\\\\,\\\\\\\\,g -e s,\',\\\\\',g
+end
+
 function __fish_abbr_get_by_key
 	if not set -q argv[1]
 		echo "__fish_abbr_get_by_key: expected one argument, got none" >&2