test: pin git fixtures to main branch, not machine default

initBareRepo (gitrepo and webhook e2e) created the initial commit on
whatever init.defaultBranch resolved to — master on machines where it is
unset — while the tests request the "main" branch, so clone-by-branch
failed with `couldn't find remote ref "refs/heads/main"`.

Pass --initial-branch=main to `git init --bare` so the bare HEAD and
initial branch are always main regardless of local git config.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: tae2089

internal/adapters/inbound/webhook/e2e_test.go

@@ -20,7 +20,7 @@ func initBareRepo(t *testing.T) string {
 	t.Helper()
 	dir := t.TempDir()
 	bareDir := filepath.Join(dir, "bare.git")
-	runTestGit(t, dir, "init", "--bare", bareDir)
+	runTestGit(t, dir, "init", "--bare", "--initial-branch=main", bareDir)
 	workDir := filepath.Join(dir, "work")
 	runTestGit(t, dir, "clone", bareDir, workDir)
 	if err := os.WriteFile(filepath.Join(workDir, "hello.txt"), []byte("hello"), 0o644); err != nil {

internal/adapters/outbound/gitrepo/checkout_test.go

@@ -27,7 +27,7 @@ func initBareRepo(t *testing.T) string {
 		}
 	}
 
-	run("init", "--bare", bareDir)
+	run("init", "--bare", "--initial-branch=main", bareDir)
 
 	workDir := filepath.Join(dir, "work")
 	run("clone", bareDir, workDir)