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>

git-merge-resolve.sh

@@ -5,6 +5,16 @@
 #
 # Resolve two trees, using enhanced multi-base read-tree.
 
+. git-sh-setup
+
+# Abort if index does not match HEAD
+if ! git diff-index --quiet --cached HEAD --
+then
+    gettextln "Error: Your local changes to the following files would be overwritten by merge"
+    git diff-index --cached --name-only HEAD -- | sed -e 's/^/    /'
+    exit 2
+fi
+
 # The first parameters up to -- are merge bases; the rest are heads.
 bases= head= remotes= sep_seen=
 for arg

t/t6424-merge-unrelated-index-changes.sh

@@ -114,6 +114,19 @@ test_expect_success 'resolve, non-trivial' '
 	test_path_is_missing .git/MERGE_HEAD
 '
 
+test_expect_success 'resolve, non-trivial, related file removed' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	git rm a &&
+	test_path_is_missing a &&
+
+	test_must_fail git merge -s resolve D^0 &&
+
+	test_path_is_missing a &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
 test_expect_success 'recursive' '
 	git reset --hard &&
 	git checkout B^0 &&