chore: always specify master as the initial git branch

Signed-off-by: Chris Fry <christopherfry@google.com>

test/e2e/fixture/fixture.go

@@ -685,7 +685,7 @@ func EnsureCleanState(t *testing.T, opts ...TestOption) {
 	// set-up tmp repo, must have unique name
 	FailOnErr(Run("", "cp", "-Rf", opt.testdata, repoDirectory()))
 	FailOnErr(Run(repoDirectory(), "chmod", "777", "."))
-	FailOnErr(Run(repoDirectory(), "git", "init"))
+	FailOnErr(Run(repoDirectory(), "git", "init", "-b", "master"))
 	FailOnErr(Run(repoDirectory(), "git", "add", "."))
 	FailOnErr(Run(repoDirectory(), "git", "commit", "-q", "-m", "initial commit"))
 
@@ -879,7 +879,7 @@ func CreateSubmoduleRepos(repoType string) {
 	// set-up submodule repo
 	FailOnErr(Run("", "cp", "-Rf", "testdata/git-submodule/", submoduleDirectory()))
 	FailOnErr(Run(submoduleDirectory(), "chmod", "777", "."))
-	FailOnErr(Run(submoduleDirectory(), "git", "init"))
+	FailOnErr(Run(submoduleDirectory(), "git", "init", "-b", "master"))
 	FailOnErr(Run(submoduleDirectory(), "git", "add", "."))
 	FailOnErr(Run(submoduleDirectory(), "git", "commit", "-q", "-m", "initial commit"))
 
@@ -891,7 +891,7 @@ func CreateSubmoduleRepos(repoType string) {
 	// set-up submodule parent repo
 	FailOnErr(Run("", "mkdir", submoduleParentDirectory()))
 	FailOnErr(Run(submoduleParentDirectory(), "chmod", "777", "."))
-	FailOnErr(Run(submoduleParentDirectory(), "git", "init"))
+	FailOnErr(Run(submoduleParentDirectory(), "git", "init", "-b", "master"))
 	FailOnErr(Run(submoduleParentDirectory(), "git", "add", "."))
 	FailOnErr(Run(submoduleParentDirectory(), "git", "submodule", "add", "-b", "master", "../submodule.git", "submodule/test"))
 	if repoType == "ssh" {