string shorten: Make max of 0 mean no shortening

This makes it easier to just slot in `string shorten` wherever,
without having to do a weird "if test $max -gt 0" check.

tests/checks/string.fish

@@ -950,3 +950,8 @@ for i in (seq 1 (string length -V -- $str))
     test $len = $i
     or echo Oopsie ellipsizing to $i failed
 end
+
+string shorten -m0 foo bar asodjsaoidj
+# CHECK: foo
+# CHECK: bar
+# CHECK: asodjsaoidj