test(check-pr-size): pin initTestRepo's initial branch so go test passes under init.defaultBranch=main

initTestRepo ran `git init -q` with no `-b`, so the fixture repo's first
branch was whatever the developer's `init.defaultBranch` happened to be.
TestResolveMergeBaseWithAdvancedBaseBranch then runs
`git checkout -q -b main <forkPoint>`, which fails with
`fatal: a branch named 'main' already exists` on any machine configured with
`init.defaultBranch = main`. CI is green only because the ubuntu-latest
runner's git still defaults to `master`.

Pin the fixture's initial branch to a sentinel name so it can never collide
with a branch the tests create themselves. `-b` needs git >= 2.28, the same
release that introduced `init.defaultBranch`, so no supported git regresses.

Author: mattmillerai

scripts/check-pr-size/main_test.go

@@ -31,7 +31,9 @@ func gitRun(t *testing.T, dir string, args ...string) string {
 func initTestRepo(t *testing.T) string {
 	t.Helper()
 	dir := t.TempDir()
-	gitRun(t, dir, "init", "-q")
+	// Pin the initial branch so the fixture never collides with a developer's
+	// init.defaultBranch (tests below create their own "main" branch).
+	gitRun(t, dir, "init", "-q", "-b", "groom-test-base")
 	gitRun(t, dir, "config", "user.email", "test@example.com")
 	gitRun(t, dir, "config", "user.name", "Test")
 	gitRun(t, dir, "config", "commit.gpgsign", "false")