checkout-index: delay automatic setting of to_tempfile

checkout-index: delay automatic setting of to_tempfile

Using --stage=all requires writing to tempfiles, since we cannot put
multiple stages into a single file. So --stage=all implies --temp.

But we do so by setting to_tempfile in the options callback for --stage,
rather than after all options have been parsed. This leads to two bugs:

  1. If you run "checkout-index --stage=all --stage=2", this should not
     imply --temp, but it currently does. The callback cannot just unset
     to_tempfile when it sees the "2" value, because it no longer knows
     if its value was from the earlier --stage call, or if the user
     specified --temp explicitly.

  2. If you run "checkout-index --stage=all --no-temp", the --no-temp
     will overwrite the earlier implied --temp. But this mode of
     operation cannot work, and the command will fail with "<path>
     already exists" when trying to write the higher stages.

We can fix both by lazily setting to_tempfile. We'll make it a tristate,
with -1 as "not yet given", and have --stage=all enable it only after
all options are parsed. Likewise, after all options are parsed we can
detect and reject the bogus "--no-temp" case.

Note that this does technically change the behavior for "--stage=all
--no-temp" for paths which have only one stage present (which
accidentally worked before, but is now forbidden). But this behavior was
never intended, and you'd have to go out of your way to try to trigger
it.

The new tests cover both cases, as well the general "--stage=all implies
--temp", as most of the other tests explicitly say "--temp". Ironically,
the test "checkout --temp within subdir" is the only one that _doesn't_
use "--temp", and so was implicitly covering this case. But it seems
reasonable to have a more explicit test alongside the other related
ones.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>