sort: make merging stable

When merging files we need to prioritize files that occur earlier in the
command line arguments with -m.

This also makes the extsort merge step (and thus extsort itself) stable again.

tests/by-util/test_sort.rs

@@ -51,6 +51,18 @@ fn test_smaller_than_specified_segment() {
         .stdout_is_fixture("ext_sort.expected");
 }
 
+#[test]
+fn test_ext_sort_stable() {
+    new_ucmd!()
+        .arg("-n")
+        .arg("--stable")
+        .arg("-S")
+        .arg("0M")
+        .arg("ext_stable.txt")
+        .succeeds()
+        .stdout_only_fixture("ext_stable.expected");
+}
+
 #[test]
 fn test_extsort_zero_terminated() {
     new_ucmd!()
@@ -566,6 +578,18 @@ fn test_merge_unique() {
         .stdout_only_fixture("merge_ints_interleaved.expected");
 }
 
+#[test]
+fn test_merge_stable() {
+    new_ucmd!()
+        .arg("-m")
+        .arg("--stable")
+        .arg("-n")
+        .arg("merge_stable_1.txt")
+        .arg("merge_stable_2.txt")
+        .succeeds()
+        .stdout_only_fixture("merge_stable.expected");
+}
+
 #[test]
 fn test_merge_reversed() {
     new_ucmd!()

tests/fixtures/sort/ext_stable.expected

@@ -0,0 +1,4 @@
+0a
+0a
+0b
+0b

tests/fixtures/sort/ext_stable.txt

@@ -0,0 +1,4 @@
+0a
+0a
+0b
+0b

tests/fixtures/sort/merge_stable.expected

@@ -0,0 +1,3 @@
+0a
+0c
+0b

tests/fixtures/sort/merge_stable_1.txt

@@ -0,0 +1,2 @@
+0a
+0c

tests/fixtures/sort/merge_stable_2.txt

@@ -0,0 +1 @@
+0b