merge: ensure we can actually restore pre-merge state

Merge strategies can:
  * succeed with a clean merge
  * succeed with a conflicted merge
  * fail to handle the given type of merge

If one is thinking in terms of automatic mergeability, they would use
the word "fail" instead of "succeed" for the second bullet, but I am
focusing here on ability of the merge strategy to handle the given
inputs, not on whether the given inputs are mergeable.  The third
category is about the merge strategy failing to know how to handle the
given data; examples include:

  * Passing more than 2 branches to 'recursive' or 'ort'
  * Passing 2 or fewer branches to 'octopus'
  * Trying to do more complicated merges with 'resolve' (I believe
    directory/file conflicts will cause it to bail.)
  * Octopus running into a merge conflict for any branch OTHER than
    the final one (see the "exit 2" codepath of commit 98efc8f3d8
    ("octopus: allow manual resolve on the last round.", 2006-01-13))

That final one is particularly interesting, because it shows that the
merge strategy can muck with the index and working tree, and THEN bail
and say "sorry, this strategy cannot handle this type of merge; use
something else".

Further, we do not currently expect the individual strategies to clean
up after themselves, but instead expect builtin/merge.c to do so.  For
it to be able to, it needs to save the state before trying the merge
strategy so it can have something to restore to.  Therefore, remove the
shortcut bypassing the save_state() call.

There is another bug on the restore_state() side of things, so no
testcase will be added until the next commit when we have addressed that
issue as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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