merge-resolve: abort if index does not match HEAD

As noted in commit 9822175d2b ("Ensure index matches head before invoking merge machinery, round N", 2019-08-17), we have had a very long history of problems with failing to enforce the requirement that index matches HEAD when starting a merge. One of the commits referenced in the long tale of issues arising from lax enforcement of this requirement was commit 55f39cf755 ("merge: fix misleading pre-merge check documentation", 2018-06-30), which tried to document the requirement and noted there were some exceptions. As mentioned in that commit message, the `resolve` strategy was the one strategy that did not have an explicit index matching HEAD check, and the reason it didn't was that I wasn't able to discover any cases where the implementation would fail to catch the problem and abort, and didn't want to introduce unnecessary performance overhead of adding another check. Well, today I discovered a testcase where the implementation does not catch the problem and so an explicit check is needed. Add a testcase that previously would have failed, and update git-merge-resolve.sh to have an explicit check. Note that the code is copied from 3ec62ad9ff ("merge-octopus: abort if index does not match HEAD", 2016-04-09), so that we reuse the same message and avoid making translators need to translate some new message. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>