add--interactive: fix external command invocation on Windows

Back in 21e9757e (Hack git-add--interactive to make it work with
ActiveState Perl, 2007-08-01), the invocation of external commands was
changed to use qx{} on Windows. The rationale was that the command
interpreter on Windows is not a POSIX shell, but rather Windows's CMD.
That patch was wrong to include 'msys' in the check whether to use qx{}
or not: 'msys' identifies MSYS perl as shipped with Git for Windows,
which does not need the special treatment; qx{} should be used only with
ActiveState perl, which is identified by 'MSWin32'.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -169,7 +169,7 @@ my %patch_modes = (
 my %patch_mode_flavour = %{$patch_modes{stage}};
 
 sub run_cmd_pipe {
-	if ($^O eq 'MSWin32' || $^O eq 'msys') {
+	if ($^O eq 'MSWin32') {
 		my @invalid = grep {m/[":*]/} @_;
 		die "$^O does not support: @invalid\n" if @invalid;
 		my @args = map { m/ /o ? "\"$_\"": $_ } @_;