legacy-rebase: backport -C<n> and --whitespace=<option> checks

Since 04519d7201 (rebase: validate -C<n> and --whitespace=<mode>
parameters early, 2018-11-14), the built-in rebase validates the -C and
--whitespace arguments early. As this commit also introduced a
regression test for this, and as a later commit introduced the
GIT_TEST_REBASE_USE_BUILTIN mode to run tests, we now have a
"regression" in the scripted version of `git rebase` on our hands.

Backport the validation to fix this.

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-legacy-rebase.sh

@@ -337,6 +337,11 @@ do
 		fix|strip)
 			force_rebase=t
 			;;
+		warn|nowarn|error|error-all)
+			;; # okay, known whitespace option
+		*)
+			die "fatal: Invalid whitespace option: '${1#*=}'"
+			;;
 		esac
 		;;
 	--ignore-whitespace)
@@ -352,6 +357,9 @@ do
 		git_am_opt="$git_am_opt $1"
 		force_rebase=t
 		;;
+	-C*[!0-9]*)
+		die "fatal: switch \`C' expects a numerical value"
+		;;
 	-C*)
 		git_am_opt="$git_am_opt $1"
 		;;