Add a test that documents the current behavior

Currently we get an annoying error message, but we'd like the base commit to be
selected, disregarding the fixup.

pkg/integration/tests/commit/find_base_commit_for_fixup_disregard_fixups_for_same_base_commit.go

@@ -0,0 +1,46 @@
+package commit
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var FindBaseCommitForFixupDisregardFixupsForSameBaseCommit = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Finds the base commit to create a fixup for, disregarding fixup commits for the same base commit",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.
+			EmptyCommit("1st commit").
+			NewBranch("mybranch").
+			CreateFileAndAdd("file1", "1\n2\n3\n").
+			Commit("2nd commit").
+			UpdateFileAndAdd("file1", "1\n2\n3a\n").
+			Commit("fixup! 2nd commit").
+			EmptyCommit("3rd commit").
+			UpdateFileAndAdd("file1", "1a\n2\n3b\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Lines(
+				Contains("3rd commit").IsSelected(),
+				Contains("fixup! 2nd commit"),
+				Contains("2nd commit"),
+				Contains("1st commit"),
+			)
+
+		t.Views().Files().
+			Focus().
+			Press(keys.Files.FindBaseCommitForFixup)
+
+		t.ExpectPopup().Alert().
+			Title(Equals("Error")).
+			Content(
+				MatchesRegexp("Multiple base commits found.*\n\n" +
+					".*fixup! 2nd commit\n" +
+					".*2nd commit"),
+			).
+			Confirm()
+	},
+})

pkg/integration/tests/test_list.go

@@ -130,6 +130,7 @@ var tests = []*components.IntegrationTest{
 	commit.DoNotShowBranchMarkerForHeadCommit,
 	commit.FailHooksThenCommitNoHooks,
 	commit.FindBaseCommitForFixup,
+	commit.FindBaseCommitForFixupDisregardFixupsForSameBaseCommit,
 	commit.FindBaseCommitForFixupDisregardMainBranch,
 	commit.FindBaseCommitForFixupOnlyAddedLines,
 	commit.FindBaseCommitForFixupWarningForAddedLines,