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

This doesn't do much for us now, but will soon be used to help us determine
what type of smartlog we should be rendering.

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

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

git-branchless/src/commands/mod.rs

@@ -340,7 +340,7 @@ fn do_main_and_drop_locals() -> eyre::Result<i32> {
             &git_run_info,
             &SmartlogOptions {
                 event_id,
-                revset,
+                revset: revset.unwrap_or_else(|| SmartlogOptions::default().revset),
                 resolve_revset_options,
             },
         )?,

git-branchless/src/commands/smartlog.rs

@@ -521,7 +521,9 @@ mod render {
         fn default() -> Self {
             Self {
                 event_id: Default::default(),
-                revset: Revset("draft() | branches() | @".to_string()),
+                revset: Revset(
+                    "((draft() | branches() | @) % main()) | branches() | @".to_string(),
+                ),
                 resolve_revset_options: Default::default(),
             }
         }

git-branchless/src/opts.rs

@@ -462,8 +462,8 @@ 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 = "draft() | branches() | @")]
-        revset: Revset,
+        #[clap(value_parser)]
+        revset: Option<Revset>,
 
         /// Options for resolving revset expressions.
         #[clap(flatten)]

git-branchless/tests/command/test_init.rs

@@ -327,7 +327,7 @@ fn test_main_branch_not_found_error_message() -> eyre::Result<()> {
 
       ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SPANTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
 
-       0: git_branchless::commands::smartlog::smartlog with effects=<Output fancy=false> git_run_info=<GitRunInfo path_to_git="<git-executable>" working_directory="<repo-path>" env=not shown> options=SmartlogOptions { event_id: None, revset: Revset("draft() | branches() | @"), resolve_revset_options: ResolveRevsetOptions { show_hidden_commits: false } }
+       0: git_branchless::commands::smartlog::smartlog with effects=<Output fancy=false> git_run_info=<GitRunInfo path_to_git="<git-executable>" working_directory="<repo-path>" env=not shown> options=SmartlogOptions { event_id: None, revset: Revset("((draft() | branches() | @) % main()) | branches() | @"), resolve_revset_options: ResolveRevsetOptions { show_hidden_commits: false } }
           at some/file/path.rs:123
 
     Suggestion: