Add a "Mark commit as base commit for rebase" command

This allows to do the equivalent of "git rebase --onto <target> <base>", by
first marking the <base> commit with the new command, and then selecting the
target branch and invoking the usual rebase command there.

pkg/gui/modes/marked_base_commit/marked_base_commit.go

@@ -0,0 +1,25 @@
+package marked_base_commit
+
+type MarkedBaseCommit struct {
+	sha string // the sha of the commit used as a rebase base commit; empty string when unset
+}
+
+func New() MarkedBaseCommit {
+	return MarkedBaseCommit{}
+}
+
+func (m *MarkedBaseCommit) Active() bool {
+	return m.sha != ""
+}
+
+func (m *MarkedBaseCommit) Reset() {
+	m.sha = ""
+}
+
+func (m *MarkedBaseCommit) SetSha(sha string) {
+	m.sha = sha
+}
+
+func (m *MarkedBaseCommit) GetSha() string {
+	return m.sha
+}