t7615: be more explicit about diff algorithm used

t7615 is entirely about testing the differences about different
diff algorithms, but it doesn't specify any diff algorithm when it
is testing myers.  Given that we have discussed potentially switching
defaults (https://lore.kernel.org/git/[email protected]/), it
makes sense in tests that are about different diff algorithms to be
explicitly about which one is intended to be used in each test.  Add
that specificity.

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

Author: newren

t/t7615-diff-algo-with-mergy-operations.sh

@@ -26,7 +26,7 @@ GIT_TEST_MERGE_ALGORITHM=recursive
 
 test_expect_success 'merge c2 to c1 with recursive merge strategy fails with the current default myers diff algorithm' '
 	git reset --hard c1 &&
-	test_must_fail git merge -s recursive c2
+	test_must_fail git merge -s recursive -Xdiff-algorithm=myers c2
 '
 
 test_expect_success 'merge c2 to c1 with recursive merge strategy succeeds with -Xdiff-algorithm=histogram' '
@@ -42,7 +42,7 @@ test_expect_success 'merge c2 to c1 with recursive merge strategy succeeds with
 
 test_expect_success 'cherry-pick c2 to c1 with recursive merge strategy fails with the current default myers diff algorithm' '
 	git reset --hard c1 &&
-	test_must_fail git cherry-pick -s recursive c2
+	test_must_fail git cherry-pick -s recursive -Xdiff-algorithm=myers c2
 '
 
 test_expect_success 'cherry-pick c2 to c1 with recursive merge strategy succeeds with -Xdiff-algorithm=histogram' '