Re: [PATCH v3 5/7] merge: make restore_state() restore staged state too

"Elijah Newren via GitGitGadget" [off-list ref] writes:

  4) Merge strategies can make changes to the index and working tree,
     and have no expectation to clean up after themselves, *even* if
     they bail out and say they are not an appropriate handler for
     the merge in question.  (The `octopus` merge strategy does this,
     for example.)

I personally consider this is a bug in `octopus` (and I can do so
without offending anybody, as `octopus` was what I did), but because
the point of having pluggable merge strategies is to allow end users
and third parties write their own.  So save-and-restore dance would
be a prudent approach to the issue than forbidding this "buggy"
behaviour.

Unfortunately, if users had staged changes before calling `git merge`,
builtin/merge.c could do the following:

   * stash the changes, in order to clean up after the strategies
   * try all the merge strategies in turn, each of which report they
     cannot function due to the index not matching HEAD
   * restore the changes via "git stash apply"

 	test_seq 0 10 >a &&
 	git add a &&
+	git rev-parse :a >expect &&
...
+	# Changes to "a" should remain staged
+	git rev-parse :a >actual &&
+	test_cmp expect actual

Makes sense.

Thanks.