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, 4 insertions, 0 deletions

diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index e6cbed2c..d7d87b5a 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -1617,6 +1617,10 @@ fn copy_selection_on_line(cx: &mut Context, direction: Direction) {
                 sels += 1;
             }
 
+            if anchor_row == 0 && head_row == 0 {
+                break;
+            }
+
             i += 1;
         }
     }