functions/history.fish: also save when called with --exact

After deleting a history item with

    history delete --exact --case-sensitive the-item

it is still reachable by history search until the shell is restarted.

Let's fix this by saving history after each deletion.  The non-exact variants
of "history delete" already do this.  I think this was just an oversight
owed to the fact that hardly anyone uses "--exact" (else we would surely
have changed it to not require an explicit "--case-sensitive").

share/functions/history.fish

@@ -120,6 +120,7 @@ function history --description "display or manipulate interactive command histor
 
             if test $search_mode = --exact
                 builtin history delete $search_mode $_flag_case_sensitive -- $searchterm
+                builtin history save
                 return
             end