Use upstream branch when opening pull requests (#2693)

- **PR Description**

Should probably solve #2691

- **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go run scripts/cheatsheet/main.go
generate`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [x] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [x] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [x] Docs (specifically `docs/Config.md`) have been updated if
necessary
* [x] You've read through your own file changes for silly mistakes etc

pkg/integration/tests/branch/open_pull_request_no_upstream.go

@@ -0,0 +1,25 @@
+package branch
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var OpenPullRequestNoUpstream = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Open up a pull request with a missing upstream branch",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo:    func(shell *Shell) {},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().
+			Branches().
+			Focus().
+			Press(keys.Branches.CreatePullRequest)
+
+		t.ExpectPopup().Alert().
+			Title(Equals("Error")).
+			Content(Contains("Cannot open a pull request for a branch with no upstream")).
+			Confirm()
+	},
+})

pkg/integration/tests/test_list.go

@@ -40,6 +40,7 @@ var tests = []*components.IntegrationTest{
 	branch.Delete,
 	branch.DeleteRemoteBranchWithCredentialPrompt,
 	branch.DetachedHead,
+	branch.OpenPullRequestNoUpstream,
 	branch.OpenWithCliArg,
 	branch.Rebase,
 	branch.RebaseAbortOnConflict,