t: fix &&-chaining issues around setup which might fail

Many tests have an initial setup step that might fail based
on whether earlier tests in the script have succeeded or
not. Using a trick like "|| true" breaks the &&-chain,
missing earlier failures (and fooling --chain-lint).

We can use test_might_fail in some cases, which is correct
and makes the intent more obvious. We can also use
test_unconfig for unsetting config (and which is more
robust, as well).

The case in t9500 is an oddball. It wants to run cmd1 _or_
cmd2, and does it like:

  cmd1 || cmd2 &&
  other_stuff

It's not wrong in this case, but it's a bad habit to get
into, because it breaks the &&-chain if used anywhere except
at the beginning of the test (and we use the correct
solution here, putting it inside a block for precedence).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t9500-gitweb-standalone-no-errors.sh b/t/t9500-gitweb-standalone-no-errors.sh
index f9f078e..e94b2f1 100755
--- a/t/t9500-gitweb-standalone-no-errors.sh
+++ b/t/t9500-gitweb-standalone-no-errors.sh
@@ -779,7 +779,10 @@
 
 test_expect_success \
 	'unborn HEAD: "summary" page (with "heads" subview)' \
-	'git checkout orphan_branch || git checkout --orphan orphan_branch &&
+	'{
+		git checkout orphan_branch ||
+		git checkout --orphan orphan_branch
+	 } &&
 	 test_when_finished "git checkout master" &&
 	 gitweb_run "p=.git;a=summary"'