t1509: make "setup" test more robust

One of the t1509 setup tests is very particular about the output it
expects from `git init`, and fails if the output differs even slightly
which can happen easily if the script is run multiple times since it
doesn't do a good job of cleaning up after itself (i.e. it leaves
detritus in the root directory `/`). One bit of cruft in particular
(`/HEAD`) makes the test fail since its presence causes `git init` to
alter its output; rather than reporting "Initialized empty Git
repository", it instead reports "Reinitialized existing Git repository"
when `/HEAD` is present. Address this problem by making the test do a
more careful job of crafting its intended initial state.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>