Add test demonstrating the desired behavior

It has two modified hunks, one for a master commit and one for a branch commit.
Currently we get an error mentioning those two commits, but we would like to
silently select the branch commit.

pkg/integration/tests/commit/find_base_commit_for_fixup_disregard_main_branch.go

@@ -0,0 +1,56 @@
+package commit
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var FindBaseCommitForFixupDisregardMainBranch = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Finds the base commit to create a fixup for, disregarding changes to a commit that is already on master",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.
+			EmptyCommit("1st commit").
+			CreateFileAndAdd("file1", "file1 content\n").
+			Commit("2nd commit").
+			NewBranch("mybranch").
+			CreateFileAndAdd("file2", "file2 content\n").
+			Commit("3rd commit").
+			EmptyCommit("4th commit").
+			UpdateFile("file1", "file1 changed content").
+			UpdateFile("file2", "file2 changed content")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Lines(
+				Contains("4th commit").IsSelected(),
+				Contains("3rd commit"),
+				Contains("2nd commit"),
+				Contains("1st commit"),
+			)
+
+		t.Views().Files().
+			Focus().
+			Press(keys.Files.FindBaseCommitForFixup)
+
+		/* EXPECTED:
+		t.Views().Commits().
+			IsFocused().
+			Lines(
+				Contains("4th commit"),
+				Contains("3rd commit").IsSelected(),
+				Contains("2nd commit"),
+				Contains("1st commit"),
+			)
+		*/
+
+		// ACTUAL:
+		t.ExpectPopup().Alert().Title(Equals("Error")).Content(
+			Contains("Multiple base commits found.").
+				Contains("2nd commit").
+				Contains("3rd commit"),
+		).Confirm()
+	},
+})