ui: fill in default term width globally and return as usize

I'm going to add more ui.term_width() callers, and it's unlikely we'll have to
set different defaults or error out.

cli/src/cli_util.rs

@@ -2139,7 +2139,7 @@ pub enum LogContentFormat {
 impl LogContentFormat {
     pub fn new(ui: &Ui, settings: &UserSettings) -> Result<Self, config::ConfigError> {
         if settings.config().get_bool("ui.log-word-wrap")? {
-            let term_width = usize::from(ui.term_width().unwrap_or(80));
+            let term_width = ui.term_width();
             Ok(LogContentFormat::Wrap { term_width })
         } else {
             Ok(LogContentFormat::NoWrap)

cli/src/diff_util.rs

@@ -257,7 +257,7 @@ impl<'a> DiffRenderer<'a> {
                 DiffFormat::Stat => {
                     let tree_diff = from_tree.diff_stream(to_tree, matcher);
                     // TODO: In graph log, graph width should be subtracted
-                    let width = usize::from(ui.term_width().unwrap_or(80));
+                    let width = ui.term_width();
                     show_diff_stat(formatter, store, tree_diff, path_converter, width)?;
                 }
                 DiffFormat::Types => {

cli/src/ui.rs

@@ -578,8 +578,8 @@ impl Ui {
         rpassword::prompt_password(format!("{prompt}: "))
     }
 
-    pub fn term_width(&self) -> Option<u16> {
-        term_width()
+    pub fn term_width(&self) -> usize {
+        term_width().unwrap_or(80).into()
     }
 }