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 <[email protected]>

Author: sunshineco

t/t1509-root-work-tree.sh

@@ -243,7 +243,7 @@ say "auto bare gitdir"
 # DESTROYYYYY!!!!!
 test_expect_success 'setup' '
 	rm -rf /refs /objects /info /hooks &&
-	rm -f /expected /ls.expected /me /result &&
+	rm -f /HEAD /expected /ls.expected /me /result &&
 	cd / &&
 	echo "Initialized empty Git repository in /" > expected &&
 	git init --bare > result &&