refactor(smartlog): Make [revset] an Option<Revset>

Unfortunately, this prevents clap from rendering the default revset in
the help message, but I believe this is the only way to detect if the
user has not not supplied an arg via the derive API.

See clap-rs/clap#3846 fmi on getting the
value_source of an arg via the derive API.

git-branchless/src/commands/smartlog.rs

@@ -509,7 +509,7 @@ mod render {
 
         /// The commits to render. These commits and their ancestors up to the
         /// main branch will be rendered.
-        pub revset: Revset,
+        pub revset: Option<Revset>,
 
         pub resolve_revset_options: ResolveRevsetOptions,
     }
@@ -560,7 +560,7 @@ pub fn smartlog(
         effects,
         &repo,
         &mut dag,
-        &[revset.clone()],
+        &[revset.clone().unwrap_or_default()],
         resolve_revset_options,
     ) {
         Ok(result) => match result.as_slice() {

git-branchless/src/opts.rs

@@ -468,8 +468,8 @@ pub enum Command {
 
         /// The commits to render. These commits and their ancestors up to the
         /// main branch will be rendered.
-        #[clap(value_parser, default_value_t)]
-        revset: Revset,
+        #[clap(value_parser)]
+        revset: Option<Revset>,
 
         /// Options for resolving revset expressions.
         #[clap(flatten)]