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/controllers/helpers/mode_helper.go

@@ -82,6 +82,16 @@ func (self *ModeHelper) Statuses() []ModeStatus {
 			},
 			Reset: self.ExitFilterMode,
 		},
+		{
+			IsActive: self.c.Modes().MarkedBaseCommit.Active,
+			Description: func() string {
+				return self.withResetButton(
+					self.c.Tr.MarkedBaseCommitStatus,
+					style.FgCyan,
+				)
+			},
+			Reset: self.mergeAndRebaseHelper.ResetMarkedBaseCommit,
+		},
 		{
 			IsActive: self.c.Modes().CherryPicking.Active,
 			Description: func() string {