Keep the same line selected after squashing fixup commits

This uses a bit of a heuristic that is hopefully correct most of the time.

pkg/gui/controllers/local_commits_controller_test.go

@@ -0,0 +1,141 @@
+package controllers
+
+import (
+	"testing"
+
+	"github.com/jesseduffield/lazygit/pkg/commands/models"
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_countSquashableCommitsAbove(t *testing.T) {
+	scenarios := []struct {
+		name           string
+		commits        []*models.Commit
+		selectedIdx    int
+		rebaseStartIdx int
+		expectedResult int
+	}{
+		{
+			name: "no squashable commits",
+			commits: []*models.Commit{
+				{Name: "abc"},
+				{Name: "def"},
+				{Name: "ghi"},
+			},
+			selectedIdx:    2,
+			rebaseStartIdx: 2,
+			expectedResult: 0,
+		},
+		{
+			name: "some squashable commits, including for the selected commit",
+			commits: []*models.Commit{
+				{Name: "fixup! def"},
+				{Name: "fixup! ghi"},
+				{Name: "abc"},
+				{Name: "def"},
+				{Name: "ghi"},
+			},
+			selectedIdx:    4,
+			rebaseStartIdx: 4,
+			expectedResult: 2,
+		},
+		{
+			name: "base commit is below rebase start",
+			commits: []*models.Commit{
+				{Name: "fixup! def"},
+				{Name: "abc"},
+				{Name: "def"},
+			},
+			selectedIdx:    1,
+			rebaseStartIdx: 1,
+			expectedResult: 0,
+		},
+		{
+			name: "base commit does not exist at all",
+			commits: []*models.Commit{
+				{Name: "fixup! xyz"},
+				{Name: "abc"},
+				{Name: "def"},
+			},
+			selectedIdx:    2,
+			rebaseStartIdx: 2,
+			expectedResult: 0,
+		},
+		{
+			name: "selected commit is in the middle of fixups",
+			commits: []*models.Commit{
+				{Name: "fixup! def"},
+				{Name: "abc"},
+				{Name: "fixup! ghi"},
+				{Name: "def"},
+				{Name: "ghi"},
+			},
+			selectedIdx:    1,
+			rebaseStartIdx: 4,
+			expectedResult: 1,
+		},
+		{
+			name: "selected commit is after rebase start",
+			commits: []*models.Commit{
+				{Name: "fixup! def"},
+				{Name: "abc"},
+				{Name: "def"},
+				{Name: "ghi"},
+			},
+			selectedIdx:    3,
+			rebaseStartIdx: 2,
+			expectedResult: 1,
+		},
+	}
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			assert.Equal(t, s.expectedResult, countSquashableCommitsAbove(s.commits, s.selectedIdx, s.rebaseStartIdx))
+		})
+	}
+}
+
+func Test_isFixupCommit(t *testing.T) {
+	scenarios := []struct {
+		subject                string
+		expectedTrimmedSubject string
+		expectedIsFixup        bool
+	}{
+		{
+			subject:                "Bla",
+			expectedTrimmedSubject: "Bla",
+			expectedIsFixup:        false,
+		},
+		{
+			subject:                "fixup Bla",
+			expectedTrimmedSubject: "fixup Bla",
+			expectedIsFixup:        false,
+		},
+		{
+			subject:                "fixup! Bla",
+			expectedTrimmedSubject: "Bla",
+			expectedIsFixup:        true,
+		},
+		{
+			subject:                "fixup! fixup! Bla",
+			expectedTrimmedSubject: "Bla",
+			expectedIsFixup:        true,
+		},
+		{
+			subject:                "amend! squash! Bla",
+			expectedTrimmedSubject: "Bla",
+			expectedIsFixup:        true,
+		},
+		{
+			subject:                "fixup!",
+			expectedTrimmedSubject: "fixup!",
+			expectedIsFixup:        false,
+		},
+	}
+	for _, s := range scenarios {
+		t.Run(s.subject, func(t *testing.T) {
+			trimmedSubject, isFixupCommit := isFixupCommit(s.subject)
+			assert.Equal(t, s.expectedTrimmedSubject, trimmedSubject)
+			assert.Equal(t, s.expectedIsFixup, isFixupCommit)
+		})
+	}
+}