formatter: reset color around newlines

There are several reasons for this:

* We can more easily skip styling a trailing blank line, which other
internal code then can correctly detect as having a trailing
newline. This fixes the TODO in tests/test_commit_template.rs.

* Some tools (like `less -R`) add an extra newline if the final
character is not a newline (e.g. if there's a color reset after
it), which led to an annoying blank line after the diff summary in
e.g. `jj status`.

* Since each line is styled independently, you get all the necessary
escapes even when grepping through the output.

* Some terminals extend background color to the end of the terminal
(i.e. past the newline character), which is probably not what the
user wanted.

* Some tools (like `less -R`) get confused and lose coloring of lines
after a newline.

src/formatter.rs

···
 use std::collections::HashMap;
 use std::io::{Error, Write};
 use std::sync::Arc;
-use std::{fmt, io};
+use std::{fmt, io, mem};
 
 use crossterm::queue;
 use crossterm::style::{Attribute, Color, SetAttribute, SetBackgroundColor, SetForegroundColor};
···
 
 impl<W: Write> Write for ColorFormatter<W> {
     fn write(&mut self, data: &[u8]) -> Result<usize, Error> {
-        self.write_new_style()?;
-        self.output.write(data)
+        /*
+        We clear the current style at the end of each line, and then we re-apply the style
+        after the newline. There are several reasons for this:
+
+         * We can more easily skip styling a trailing blank line, which other
+           internal code then can correctly detect as having a trailing
+           newline.
+
+         * Some tools (like `less -R`) add an extra newline if the final
+           character is not a newline (e.g. if there's a color reset after
+           it), which led to an annoying blank line after the diff summary in
+           e.g. `jj status`.
+
+         * Since each line is styled independently, you get all the necessary
+           escapes even when grepping through the output.
+
+         * Some terminals extend background color to the end of the terminal
+           (i.e. past the newline character), which is probably not what the
+           user wanted.
+
+         * Some tools (like `less -R`) get confused and lose coloring of lines
+           after a newline.
+         */
+        for line in data.split_inclusive(|b| *b == b'\n') {
+            if line.ends_with(b"\n") {
+                self.write_new_style()?;
+                self.output.write_all(&line[..line.len() - 1])?;
+                let labels = mem::take(&mut self.labels);
+                self.write_new_style()?;
+                self.output.write_all(b"\n")?;
+                self.labels = labels;
+            } else {
+                self.write_new_style()?;
+                self.output.write_all(line)?;
+            }
+        }
+        Ok(data.len())
     }
 
     fn flush(&mut self) -> Result<(), Error> {

tests/test_commit_template.rs

···
     // Color
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "--color=always"]);
     insta::assert_snapshot!(stdout, @r###"
-    @ ffdaa62087a2 [38;5;[e-mail] 2001-02-03 04:05:09.000 +07:00 my-branch 9de54178d59d
-    | description 1
-    | 
+    @ ffdaa62087a2 [38;5;[e-mail] 2001-02-03 04:05:09.000 +07:00 my-branch 9de54178d59d
+    | description 1
     o 9a45c67d3e96 [38;5;[e-mail] 2001-02-03 04:05:08.000 +07:00 4291e264ae97
     | add a file
     o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
···
     // Color without graph
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "--color=always", "--no-graph"]);
     insta::assert_snapshot!(stdout, @r###"
-    ffdaa62087a2 [38;5;[e-mail] 2001-02-03 04:05:09.000 +07:00 my-branch 9de54178d59d
-    description 1
-    9a45c67d3e96 [38;5;[e-mail] 2001-02-03 04:05:08.000 +07:00 4291e264ae97
+    ffdaa62087a2 [38;5;[e-mail] 2001-02-03 04:05:09.000 +07:00 my-branch 9de54178d59d
+    description 1
+    9a45c67d3e96 [38;5;[e-mail] 2001-02-03 04:05:08.000 +07:00 4291e264ae97
     add a file
     000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
     (no description set)
···
     "###);
 
     // Color
-    // TODO: We shouldn't get a blank line after "description 1"
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "--color=always"]);
     insta::assert_snapshot!(stdout, @r###"
     o 9a45c67d3e96?? [38;5;[e-mail] 2001-02-03 04:05:10.000 +07:00 8979953d4c67
     | description 2
-    | @ 9a45c67d3e96?? [38;5;[e-mail] 2001-02-03 04:05:08.000 +07:00 7a17d52e633c
-    |/  description 1
-    |   
+    | @ 9a45c67d3e96?? [38;5;[e-mail] 2001-02-03 04:05:08.000 +07:00 7a17d52e633c
+    |/  description 1
     o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
       (no description set)
     "###);

tests/test_log_command.rs

···
     o (no description set)
     "###);
     let stdout = test_env.jj_cmd_success(&repo_path, &["--color=always", "log", "-T=description"]);
-    // TODO: The color codes shouldn't span the graph lines, and we shouldn't get an
-    // extra line at the end
     insta::assert_snapshot!(stdout, @r###"
-    @ single line
-    | 
-    o first line
-    | second line
-    | third line
-    | 
-    o (no description set)
-      
+    @ single line
+    o first line
+    | second line
+    | third line
+    o (no description set)
     "###);
 }