ui: write prompt messages to stderr

This enables more scripting use cases where the caller may wish to parse stdout for programmatic purposes.

cli/src/ui.rs

@@ -560,7 +560,7 @@ impl Ui {
     }
 
     pub fn can_prompt() -> bool {
-        io::stdout().is_terminal()
+        io::stderr().is_terminal()
             || env::var("JJ_INTERACTIVE")
                 .map(|v| v == "1")
                 .unwrap_or(false)
@@ -573,8 +573,8 @@ impl Ui {
                 "Cannot prompt for input since the output is not connected to a terminal",
             ));
         }
-        write!(self.stdout(), "{prompt}: ")?;
-        self.stdout().flush()?;
+        write!(self.stderr(), "{prompt}: ")?;
+        self.stderr().flush()?;
         let mut buf = String::new();
         io::stdin().read_line(&mut buf)?;
 
@@ -601,7 +601,7 @@ impl Ui {
         if !Self::can_prompt() {
             if let Some(default) = default {
                 // Choose the default automatically without waiting.
-                writeln!(self.stdout(), "{prompt}: {default}")?;
+                writeln!(self.stderr(), "{prompt}: {default}")?;
                 return Ok(default.to_owned());
             }
         }

cli/tests/test_next_prev_commands.rs

@@ -393,10 +393,9 @@ fn test_next_fails_on_bookmarking_children_quit_prompt() {
     1: zsuskuln 5f24490d (empty) third
     2: rlvkpnrz 9ed53a4a (empty) second
     q: quit the prompt
-    enter the index of the commit you want to target: 
     "###);
     insta::assert_snapshot!(stderr,@r###"
-    Error: ambiguous target commit
+    enter the index of the commit you want to target: Error: ambiguous target commit
     "###);
 }
 
@@ -420,10 +419,9 @@ fn test_next_choose_bookmarking_child() {
     2: zsuskuln 5f24490d (empty) third
     3: rlvkpnrz 9ed53a4a (empty) second
     q: quit the prompt
-    enter the index of the commit you want to target: 
     "###);
     insta::assert_snapshot!(stderr,@r###"
-    Working copy now at: yostqsxw 5c8fa96d (empty) (no description set)
+    enter the index of the commit you want to target: Working copy now at: yostqsxw 5c8fa96d (empty) (no description set)
     Parent commit      : zsuskuln 5f24490d (empty) third
     "###);
 }
@@ -463,10 +461,9 @@ fn test_prev_on_merge_commit() {
     1: zsuskuln b0d21db3 right | (empty) second
     2: qpvuntsm fa15625b left | (empty) first
     q: quit the prompt
-    enter the index of the commit you want to target: 
     "###);
     insta::assert_snapshot!(stderr,@r###"
-    Working copy now at: qpvuntsm fa15625b left | (empty) first
+    enter the index of the commit you want to target: Working copy now at: qpvuntsm fa15625b left | (empty) first
     Parent commit      : zzzzzzzz 00000000 (empty) (no description set)
     "###);
 }
@@ -509,10 +506,9 @@ fn test_prev_on_merge_commit_with_parent_merge() {
     2: qpvuntsm 6799aaa2 (empty) x
     3: zzzzzzzz 00000000 (empty) (no description set)
     q: quit the prompt
-    enter the index of the commit you want to target: 
     "###);
     insta::assert_snapshot!(stderr,@r###"
-    Working copy now at: vruxwmqv e5a6794c (empty) (no description set)
+    enter the index of the commit you want to target: Working copy now at: vruxwmqv e5a6794c (empty) (no description set)
     Parent commit      : qpvuntsm 6799aaa2 (empty) x
     "###);
 
@@ -523,10 +519,9 @@ fn test_prev_on_merge_commit_with_parent_merge() {
     1: mzvwutvl 89b8a355 (empty) 1
     2: zsuskuln a83fc061 (empty) z
     q: quit the prompt
-    enter the index of the commit you want to target: 
     "###);
     insta::assert_snapshot!(stderr,@r###"
-    Working copy now at: zsuskuln a83fc061 (empty) z
+    enter the index of the commit you want to target: Working copy now at: zsuskuln a83fc061 (empty) z
     Parent commit      : qpvuntsm 6799aaa2 (empty) x
     Parent commit      : kkmpptxz 146d5c67 (empty) y
     "###);
@@ -569,10 +564,9 @@ fn test_prev_prompts_on_multiple_parents() {
     2: kkmpptxz b0d21db3 (empty) second
     3: qpvuntsm fa15625b (empty) first
     q: quit the prompt
-    enter the index of the commit you want to target: 
     "###);
     insta::assert_snapshot!(stderr,@r###"
-    Working copy now at: kpqxywon ddac00b0 (empty) (no description set)
+    enter the index of the commit you want to target: Working copy now at: kpqxywon ddac00b0 (empty) (no description set)
     Parent commit      : qpvuntsm fa15625b (empty) first
     "###);