fix(file-tree): fold a filename's line breaks where it is drawn

A filename is bytes to the kernel, so `touch $'a\nb'` makes a file the
tree has to draw, and gpui breaks text on `\n` whatever the row says.
The row grows to two lines, its `text_ellipsis` contract stops holding,
and the rows below it slide down — one odd file misaligns the column.

The terminal's history rows already met this and already carry the fold
(`one_line`), so reuse it rather than spell a second one. What differs
here is where it may be applied: a history entry is only ever drawn, but
a filename is also handed to `join`, `rename` and `remove`. Folding at
ingestion would splice a `↵` into a name that has to match what is on
disk, so the fold sits at the two `.child(...)` sites and nothing on the
operations path sees it.

src/terminal/view.rs

@@ -6213,7 +6213,16 @@ fn one_line_char(c: char) -> char {
     }
 }
 
-fn one_line(text: &str) -> String {
+/// `text` with every line break and control character folded to a visible
+/// stand-in, so it draws on exactly one row.
+///
+/// The hazard is not this module's alone. Anything that draws a name it did
+/// not compose is exposed to it — a filename is bytes to the kernel, so
+/// `touch $'a\nb'` is a file the tree has to draw. Fold at the point of
+/// drawing, never at the point of reading: the unfolded name is what
+/// `join`/`rename`/`remove` are handed, and a `↵` spliced into it names
+/// nothing on disk.
+pub(crate) fn one_line(text: &str) -> String {
     text.chars().map(one_line_char).collect()
 }
 
@@ -6766,6 +6775,25 @@ mod tests {
         assert_eq!(unescape_mark_text("cargo build"), "cargo build");
     }
 
+    #[test]
+    fn a_name_drawn_on_one_row_keeps_its_line_breaks_visible() {
+        use super::one_line;
+        // A newline is the whole point: gpui breaks on it whatever the row
+        // says, so it has to become something that occupies a column instead.
+        assert_eq!(one_line("a\nb"), "a↵b");
+        assert_eq!(one_line("a\r\nb"), "a ↵b");
+        // Every other control character folds to a space rather than
+        // vanishing, so the name's length still reflects what is on disk.
+        assert_eq!(one_line("a\tb\u{7}c"), "a b c");
+        // One char in, one char out — a fuzzy matcher's byte positions are
+        // handed to this and still have to address the same characters.
+        for s in ["plain.rs", "a\nb", "a\tb", "日本語\n"] {
+            assert_eq!(one_line(s).chars().count(), s.chars().count(), "{s:?}");
+        }
+        // Text with nothing to fold comes back untouched.
+        assert_eq!(one_line("README.md"), "README.md");
+    }
+
     #[test]
     fn a_busy_command_name_stays_short_enough_to_read() {
         use super::{BUSY_COMMAND_MAX, clamp_command};

src/ui/file_tree.rs

@@ -6,6 +6,7 @@ use std::sync::Arc;
 use crate::core::config::RightPanelTab;
 use crate::core::git::status::{DecoStatus, DirRollup, StatusIndex};
 use crate::terminal::git_data::index_of;
+use crate::terminal::view::one_line;
 use crate::ui::app::Tty7App;
 use crate::ui::file_copy;
 use crate::ui::host_ops::{ByHost, HostId, HostOps, InFlight, SharedHost, WatchSub};
@@ -1815,7 +1816,7 @@ impl Tty7App {
                 .when(deco.strike, |d| d.line_through())
                 .when(deco.bold, |d| d.font_weight(gpui::FontWeight::SEMIBOLD))
                 .when(row.is_root, |d| d.font_weight(gpui::FontWeight::MEDIUM))
-                .child(SharedString::from(row.entry.name.clone()))
+                .child(SharedString::from(one_line(&row.entry.name)))
                 .into_any_element()
         };
 
@@ -2117,7 +2118,7 @@ impl gpui::Render for DragGhost {
             .border_color(cx.theme().border)
             .text_sm()
             .child(Icon::new(IconName::File).size(px(ROW_GLYPH)))
-            .child(SharedString::from(self.name.clone()))
+            .child(SharedString::from(one_line(&self.name)))
     }
 }