renderdag: skip separator line if rendered >1 lines for a row

Summary:
Intended to fix the issue that D107317575 is fixing. But in a more general
way.

D107317575 only considered ending with blank lines. A more accurate test is if
the previous row produces multiple lines.

___

Differential Revision: D107434238

fbshipit-source-id: c8ed6b8455c0d7e9099237adf29999e2cc22e9eb

Author: quark-zju

eden/scm/lib/renderdag/src/box_drawing.rs

@@ -238,7 +238,6 @@ mod tests {
             o  A"#
         );
 
-        // Suboptimal: extra blank line after C is unnecessary.
         assert_eq!(
             render(&TestFixture {
                 messages: &[("C", "line 1\nline 2\n")],
@@ -249,7 +248,6 @@ mod tests {
             o  C
                line 1
                line 2
-            
             o  B
             
             o  A"#

eden/scm/lib/renderdag/src/output.rs

@@ -26,15 +26,20 @@ pub struct OutputRendererOptions {
 #[derive(Default)]
 pub(crate) struct OutputRendererState {
     queued_pad_line: Option<String>,
-    last_line_is_blank: bool,
+    /// If > 1, no need for an extra separator for the next row.
+    /// Separator is only needed for adjacent single-line node lines.
+    /// If there are link, pad, term, or other kinds of node lines,
+    /// no need to draw separator.
+    row_height: usize,
 }
 
 impl OutputRendererState {
     pub(crate) fn begin_row(&mut self, out: &mut String, separator_line: bool) {
         self.flush_queued_pad_line(out);
-        if separator_line {
-            self.maybe_push_separator_line(out);
+        if separator_line && self.row_height == 1 {
+            self.push_line(out, "");
         }
+        self.row_height = 0;
     }
 
     pub(crate) fn push_line_with_message(
@@ -75,14 +80,7 @@ impl OutputRendererState {
     fn push_line(&mut self, out: &mut String, line: &str) {
         out.push_str(line.trim_end());
         out.push('\n');
-        self.last_line_is_blank = line.trim_end().is_empty();
-    }
-
-    fn maybe_push_separator_line(&mut self, out: &mut String) {
-        if !self.last_line_is_blank {
-            out.push('\n');
-            self.last_line_is_blank = true;
-        }
+        self.row_height = self.row_height.saturating_add(1);
     }
 
     fn flush_queued_pad_line(&mut self, out: &mut String) {