p9210: fix 'scalar clone' when running from a detached HEAD

In p9210-scalar-clone.sh, we test using 'scalar clone' to clone
$GIT_PERF_LARGE_REPO (copied locally as 'to-clone'), which defaults to
the git.git checkout we are running the test from.

When --branch is not specified (as in this test), 'scalar clone' tries
to get the default branch of the remote repository by parsing the output
of 'git ls-remote --symref $URL HEAD', as implemented in
scalar.c:remote_default_branch. When the git.git checkout we are running
the test from is in detached HEAD, this fails and we fall back to using
the name of the currently checked out branch in the newly initialized
repository, which in this case is the value returned earlier in
cmd_clone by repo_default_branch_name.

We then invoke 'git checkout -t origin/$branch', with $branch being the
name we got from remote_default_branch. This invocation fails if
'$branch' does not exist as a branch in the current git.git checkout.

Fix this by creating a local branch in 'to-clone' in the setup test
"enable server-side partial clone", making sure to use '-B' in case a
branch named 'test-branch' already exists.

Signed-off-by: Philippe Blain <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phil-blain

t/perf/p9210-scalar.sh

@@ -7,7 +7,8 @@ test_perf_large_repo "$TRASH_DIRECTORY/to-clone"
 
 test_expect_success 'enable server-side partial clone' '
 	git -C to-clone config uploadpack.allowFilter true &&
-	git -C to-clone config uploadpack.allowAnySHA1InWant true
+	git -C to-clone config uploadpack.allowAnySHA1InWant true &&
+	git -C to-clone checkout -B test-branch
 '
 
 test_perf 'scalar clone' '