nd/magic-pathspec exposes breakage in git-add--interactive on Windows

[Johannes Sixt <j.sixt@viscovery.net>]

With nd/magic-pathspec I get the following failure on Windows in
t2016-checkout-patch.sh:

expecting success:
        set_state dir/foo work head &&
        # the third n is to get out in case it mistakenly does not apply
        (echo y; echo n; echo n) | (cd dir && git checkout -p foo) &&
        verify_saved_state bar &&
        verify_state dir/foo head head

==== xx.
==== 1d.
==== 10e.
msys does not support: :(prefix:4)dir/foo
not ok 13 - path limiting works: foo inside dir

The error message 'msys does not support...' originates from this function
in git-add--interactive.perl (which is invoked from checkout -p):

  sub run_cmd_pipe {
	if ($^O eq 'MSWin32' || $^O eq 'msys') {
		my @invalid = grep {m/[":*]/} @_;
		die "$^O does not support: @invalid\n" if @invalid;
		my @args = map { m/ /o ? "\"$_\"": $_ } @_;
		return qx{@args};
	} else {
		my $fh = undef;
		open($fh, '-|', @_) or die;
		return <$fh>;
	}
  }

It looks like on Windows we disallow arguments that contain double-quote,
colon, or asterisk, and otherwise wrap arguments in double-quotes if they
contain space. Then pass them through qx{}, which I can only guess what it
does.

This code was introduced in 21e9757e (Hack git-add--interactive to make it
work with ActiveState Perl, 2007-08-01). The commit message has the
general statement "It wont work for arguments with special characters
(like ", : or *)), which I do not know how to interpret: Does ActiveState
Perl not work with special charactoers, or Windows? Because the latter is
definitely not true.

Can we be more permissive in the 'my @invalid' check without breaking
ActiveState Perl?

BTW, there is a similar failure in t7105-reset-patch.sh, which invokes
'git reset -p', but I haven't investigate further.

-- Hannes