t6044: add more testcases with staged changes before a merge is invoked

According to Documentation/git-merge.txt,

    ...[merge will] abort if there are any changes registered in the index
    relative to the `HEAD` commit.  (One exception is when the changed
    index entries are in the state that would result from the merge
    already.)

Add some tests showing that this exception, while it does accurately state
what would be a safe condition under which we could allow the merge to
proceed, is not what is actually implemented.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

t/t6044-merge-unrelated-index-changes.sh

@@ -137,6 +137,35 @@ test_expect_success 'merge-recursive, when index==head but head!=HEAD' '
 	test_i18ngrep "Already up to date" out
 '
 
+test_expect_failure 'recursive, when file has staged changes not matching HEAD nor what a merge would give' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	mkdir subdir &&
+	test_seq 1 10 >subdir/a &&
+	git add subdir/a &&
+
+	# HEAD has no subdir/a; merge would write 1..11 to subdir/a;
+	# Since subdir/a matches neither HEAD nor what the merge would write
+	# to that file, the merge should fail to avoid overwriting what is
+	# currently found in subdir/a
+	test_must_fail git merge -s recursive E^0
+'
+
+test_expect_failure 'recursive, when file has staged changes matching what a merge would give' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	mkdir subdir &&
+	test_seq 1 11 >subdir/a &&
+	git add subdir/a &&
+
+	# HEAD has no subdir/a; merge would write 1..11 to subdir/a;
+	# Since subdir/a matches what the merge would write to that file,
+	# the merge should be safe to proceed
+	git merge -s recursive E^0
+'
+
 test_expect_success 'octopus, unrelated file touched' '
 	git reset --hard &&
 	git checkout B^0 &&