Improve readability of Jenkins.getTarget()

The ~dozen inlined options and braceless conditional weren't the easiest
to read, especially with the one compound statement in there. Took me a
minute to convince myself that operator precedence & associativity
really made that work as intended.

I *think* those strings could drop their slashes and be combined with
the other loop (e.g. go over
Sets.concat(ALWAYS_READABLE_PATHS, getUnprotectedRootActions()), but
without spending more time confirming that, I'm leaving the behavior
exactly the same as it was before.