cat: slightly change warning message about non-file paths

I'm going to add a similar message for access denied. That will want
an error message printed at the end. For consistency, let's do the
same for non-file paths.

cli/src/commands/cat.rs

@@ -114,7 +114,7 @@ fn write_tree_entries<P: AsRef<RepoPath>>(
                 let ui_path = workspace_command.format_file_path(path.as_ref());
                 writeln!(
                     ui.warning_default(),
-                    "Path exists but is not a file: {ui_path}"
+                    "Path '{ui_path}' exists but is not a file"
                 )?;
             }
             MaterializedTreeValue::Tree(_) => panic!("entries should not contain trees"),

cli/tests/test_cat_command.rs

@@ -118,6 +118,6 @@ fn test_cat_symlink() {
     a
     "###);
     insta::assert_snapshot!(stderr, @r###"
-    Warning: Path exists but is not a file: symlink1
+    Warning: Path 'symlink1' exists but is not a file
     "###);
 }