Add HeadCommitIdx helper function

Not used yet, we'll need it in the next commit.

pkg/commands/models/commit.go

@@ -160,3 +160,13 @@ func (c *Commit) IsTODO() bool {
 func IsHeadCommit(commits []*Commit, index int) bool {
 	return !commits[index].IsTODO() && (index == 0 || commits[index-1].IsTODO())
 }
+
+func HeadCommitIdx(commits []*Commit) int {
+	for index, commit := range commits {
+		if !commit.IsTODO() {
+			return index
+		}
+	}
+
+	return -1
+}

pkg/commands/models/commit_test.go

@@ -8,6 +8,49 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestHeadCommitIdx(t *testing.T) {
+	testCases := []struct {
+		name     string
+		commits  []*Commit
+		expected int
+	}{
+		{
+			name:     "first commit without rebase todos",
+			commits:  makeTestCommits("a", "b"),
+			expected: 0,
+		},
+		{
+			name: "first non-todo commit during an interactive rebase",
+			commits: []*Commit{
+				makeTestTodoCommit(todo.Pick),
+				makeTestTodoCommit(todo.Reword),
+				makeTestCommit("a"),
+				makeTestCommit("b"),
+			},
+			expected: 2,
+		},
+		{
+			name:     "no commits",
+			commits:  nil,
+			expected: -1,
+		},
+		{
+			name: "only rebase todos",
+			commits: []*Commit{
+				makeTestTodoCommit(todo.Pick),
+				makeTestTodoCommit(todo.Reword),
+			},
+			expected: -1,
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			assert.Equal(t, testCase.expected, HeadCommitIdx(testCase.commits))
+		})
+	}
+}
+
 func TestIsHeadCommit(t *testing.T) {
 	commits := []*Commit{
 		makeTestTodoCommit(todo.Pick),
@@ -20,6 +63,15 @@ func TestIsHeadCommit(t *testing.T) {
 	assert.False(t, IsHeadCommit(commits, 2))
 }
 
+func makeTestCommits(hashes ...string) []*Commit {
+	commits := make([]*Commit, 0, len(hashes))
+	for _, hash := range hashes {
+		commits = append(commits, makeTestCommit(hash))
+	}
+
+	return commits
+}
+
 func makeTestCommit(hash string) *Commit {
 	return NewCommit(&utils.StringPool{}, NewCommitOpts{Hash: hash})
 }