Fix the infinite loop when copying the cursor to the top of the file (#5888)

Example:
```
test
testitem
```

Select line 2 with x, then type Alt-C; Helix will go into an infinite
loop. The saturating_sub keeps the head_row and anchor_row pinned at 0,
and a selection is never made since the first line is too short.

1 files changed, 64 insertions, 0 deletions

diff --git a/helix-term/tests/test/commands.rs b/helix-term/tests/test/commands.rs
index 2ca9e395..e8d16bfa 100644
--- a/helix-term/tests/test/commands.rs
+++ b/helix-term/tests/test/commands.rs
@@ -132,6 +132,70 @@ async fn test_selection_duplication() -> anyhow::Result<()> {
         .as_str(),
     ))
     .await?;
+
+    // Copy the selection to previous line, skipping the first line in the file
+    test((
+        platform_line(indoc! {"\
+            test
+            #[testitem|]#
+            "})
+        .as_str(),
+        "<A-C>",
+        platform_line(indoc! {"\
+            test
+            #[testitem|]#
+            "})
+        .as_str(),
+    ))
+    .await?;
+
+    // Copy the selection to previous line, including the first line in the file
+    test((
+        platform_line(indoc! {"\
+            test
+            #[test|]#
+            "})
+        .as_str(),
+        "<A-C>",
+        platform_line(indoc! {"\
+            #[test|]#
+            #(test|)#
+            "})
+        .as_str(),
+    ))
+    .await?;
+
+    // Copy the selection to next line, skipping the last line in the file
+    test((
+        platform_line(indoc! {"\
+            #[testitem|]#
+            test
+            "})
+        .as_str(),
+        "C",
+        platform_line(indoc! {"\
+            #[testitem|]#
+            test
+            "})
+        .as_str(),
+    ))
+    .await?;
+
+    // Copy the selection to next line, including the last line in the file
+    test((
+        platform_line(indoc! {"\
+            #[test|]#
+            test
+            "})
+        .as_str(),
+        "C",
+        platform_line(indoc! {"\
+            #(test|)#
+            #[test|]#
+            "})
+        .as_str(),
+    ))
+    .await?;
     Ok(())
 }