Remove mainBranch parameter from Shell.Init()

For older git versions we won't be able to support any other main branch than
"master", so hard-code that in Init.

This doesn't fix anything for older versions yet; see the next commit for that.

pkg/integration/components/runner.go

@@ -141,7 +141,7 @@ func buildLazygit() error {
 
 func createFixture(test *IntegrationTest, paths Paths, rootDir string) error {
 	shell := NewShell(paths.ActualRepo(), func(errorMsg string) { panic(errorMsg) })
-	shell.Init("master")
+	shell.Init()
 
 	os.Setenv(GIT_CONFIG_GLOBAL_ENV_VAR, globalGitConfigPath(rootDir))