Add test demonstrating a bug with preserving the commit message

SplitCommitMessageAndDescription splits at the first '\n\n' that it finds (if
there is one), which in this case is between the two paragraphs of the
description. This is wrong.

pkg/integration/tests/commit/preserve_commit_message.go

@@ -0,0 +1,48 @@
+package commit
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var PreserveCommitMessage = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Test that the commit message is preserved correctly when canceling the commit message panel",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateFileAndAdd("myfile", "myfile content")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Files.CommitChanges)
+
+		t.ExpectPopup().CommitMessagePanel().
+			InitialText(Equals("")).
+			Type("my commit message").
+			SwitchToDescription().
+			Type("first paragraph").
+			AddNewline().
+			AddNewline().
+			Type("second paragraph").
+			Cancel()
+
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Files.CommitChanges)
+
+		/* EXPECTED:
+		t.ExpectPopup().CommitMessagePanel().
+			Content(Equals("my commit message")).
+			SwitchToDescription().
+			Content(Equals("first paragraph\n\nsecond paragraph"))
+
+		   ACTUAL:
+		*/
+		t.ExpectPopup().CommitMessagePanel().
+			Content(Equals("my commit message\nfirst paragraph")).
+			SwitchToDescription().
+			Content(Equals("second paragraph"))
+	},
+})