merge: only apply autostash when appropriate

merge: only apply autostash when appropriate

If a merge failed and we are leaving conflicts in the working directory
for the user to resolve, we should not attempt to apply any autostash.

Further, if we fail to apply the autostash (because either the merge
failed, or the user requested --no-commit), then we should instruct the
user how to apply it later.

Add a testcase verifying we have corrected this behavior.

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