do not execute external command in pkill completions (#4586)

Running "cut" multiple times in a loop has an adverse performance
impact on first use, especially on slow systems. Using builtin "read"
for the same purpose is faster and cleaner.

share/completions/pkill.fish

@@ -2,8 +2,7 @@
 __fish_complete_pgrep pkill
 __fish_make_completion_signals
 for i in $__kill_signals
-    set number (echo $i | cut -d " " -f 1)
-    set name (echo $i | cut -d " " -f 2)
+    echo $i | read number name
     complete -c pkill -o $number -d $name
     complete -c pkill -o $name -d $name
 end