t2024: fix loose/strict local base branch DWIM test

Test 'loosely defined local base branch is reported correctly' in
t2024-checkout-dwim.sh, which was introduced in [1] compares output of
two invocations of "git checkout", invoked with two different branches
named "strict" and "loose".  As per description in [1], the test is
validating that output of tracking information for these two branches.
This tracking information is printed to standard output:

    Your branch is behind 'main' by 1 commit, and can be fast-forwarded.
      (use "git pull" to update your local branch)

The test assumes that the names of the two branches (strict and loose)
are in that output, and pipes the output through sed to replace names of
the branches with "BRANCHNAME".  Command "git checkout", however,
outputs the branch name to standard error, not standard output -- see
message "Switched to branch '%s'\n" in function "update_refs_for_switch"
in "builtin/checkout.c".  This means that the two invocations of sed do
nothing.

Redirect both the standard output and the standard error of "git
checkout" for these assertions.  Ensure that compared files have the
string "BRANCHNAME".

In a series of piped commands, only the return code of the last command
is used.  Thus, all other commands will have their return codes masked.
Avoid piping of output of git directly into sed to preserve the exit
status code of "git checkout", while we're here.

[1] 05e73682cd (checkout: report upstream correctly even with loosely
    defined branch.*.merge, 2014-10-14)

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2024-checkout-dwim.sh

@@ -305,10 +305,13 @@ test_expect_success 'loosely defined local base branch is reported correctly' '
 	test_config branch.strict.merge refs/heads/main &&
 	test_config branch.loose.merge main &&
 
-	git checkout strict | sed -e "s/strict/BRANCHNAME/g" >expect &&
+	git checkout strict >expect.raw 2>&1 &&
+	sed -e "s/strict/BRANCHNAME/g" <expect.raw >expect &&
 	status_uno_is_clean &&
-	git checkout loose | sed -e "s/loose/BRANCHNAME/g" >actual &&
+	git checkout loose >actual.raw 2>&1 &&
+	sed -e "s/loose/BRANCHNAME/g" <actual.raw >actual &&
 	status_uno_is_clean &&
+	grep BRANCHNAME actual &&
 
 	test_cmp expect actual
 '