feat: draw a subset of the full graph by specifying refspecs (#143)

## Summary

- Add positional  argument to scope the graph to specific branches
- When multiple refspecs are given, the graph is bounded by their
merge-base at the bottom and the branch tips at the top
- A single refspec with an upstream tracking branch auto-detects the
fork point
- No changes to default behavior when no refspecs are specified

## Example

```console
$ git-graph master A B
 ○<──┐  cf43da7 (HEAD -> master) Merge branch A into master
 ○<┐ │  ddcfb2d Merge branch B into master
 │ ● │  bcfb676 (B) ...
 │ │ ●  019ae78 (A) ...
 ├─┴─┘
 ○      9cca380 Merge branch X into master
```

## Lazygit integration

```yaml
git:
  branchLogCmd: "git-graph --color always {{branchName}}"
```

Closes #55

Author: aimuzov

src/graph.rs

@@ -45,6 +45,7 @@ impl GitGraph {
         settings: &Settings,
         start_point: Option<String>,
         max_count: Option<usize>,
+        refspecs: Vec<String>,
     ) -> Result<Self, String> {
         #![doc = include_str!("../docs/branch_assignment.md")]
         let mut stashes = HashSet::new();
@@ -62,17 +63,7 @@ impl GitGraph {
         walk.set_sorting(git2::Sort::TOPOLOGICAL | git2::Sort::TIME)
             .map_err(|err| err.message().to_string())?;
 
-        // Use starting point if specified
-        if let Some(start) = start_point {
-            let object = repository
-                .revparse_single(&start)
-                .map_err(|err| format!("Failed to resolve start point '{}': {}", start, err))?;
-            walk.push(object.id())
-                .map_err(|err| err.message().to_string())?;
-        } else {
-            walk.push_glob("*")
-                .map_err(|err| err.message().to_string())?;
-        }
+        configure_revwalk(&repository, &mut walk, start_point, &refspecs)?;
 
         if repository.is_shallow() {
             return Err("ERROR: git-graph does not support shallow clones due to a missing feature in the underlying libgit2 library.".to_string());
@@ -300,6 +291,86 @@ impl BranchVis {
     }
 }
 
+/// For a single refspec, find a base branch to compare against
+/// using the branch's upstream tracking ref.
+fn find_base_oid(repository: &Repository, refspec: &str, tip_oid: Oid) -> Option<Oid> {
+    if let Ok(branch) = repository.find_branch(refspec, BranchType::Local) {
+        if let Ok(upstream) = branch.upstream() {
+            if let Some(oid) = upstream.get().target() {
+                if oid != tip_oid {
+                    return Some(oid);
+                }
+            }
+        }
+    }
+
+    None
+}
+
+fn hide_ancestors_of(repository: &Repository, walk: &mut git2::Revwalk, merge_base: Oid) {
+    if let Ok(commit) = repository.find_commit(merge_base) {
+        for parent in commit.parents() {
+            let _ = walk.hide(parent.id());
+        }
+    }
+}
+
+fn configure_revwalk(
+    repository: &Repository,
+    walk: &mut git2::Revwalk,
+    start_point: Option<String>,
+    refspecs: &[String],
+) -> Result<(), String> {
+    if !refspecs.is_empty() {
+        let mut resolved_oids = Vec::with_capacity(refspecs.len());
+        for refspec in refspecs {
+            let object = repository
+                .revparse_single(refspec)
+                .map_err(|err| format!("Failed to resolve refspec '{}': {}", refspec, err))?;
+            let oid = object.id();
+            walk.push(oid).map_err(|err| err.message().to_string())?;
+            resolved_oids.push(oid);
+        }
+
+        if resolved_oids.len() == 1 {
+            // Single refspec: auto-detect base branch
+            if let Some(base_oid) = find_base_oid(repository, &refspecs[0], resolved_oids[0]) {
+                walk.push(base_oid)
+                    .map_err(|err| err.message().to_string())?;
+                if let Ok(mb) = repository.merge_base(resolved_oids[0], base_oid) {
+                    hide_ancestors_of(repository, walk, mb);
+                }
+            }
+        } else {
+            // Multiple refspecs: compute merge-base of all
+            let mut base = resolved_oids[0];
+            let mut base_found = true;
+            for oid in &resolved_oids[1..] {
+                match repository.merge_base(base, *oid) {
+                    Ok(mb) => base = mb,
+                    Err(_) => {
+                        base_found = false;
+                        break;
+                    }
+                }
+            }
+            if base_found {
+                hide_ancestors_of(repository, walk, base);
+            }
+        }
+    } else if let Some(start) = start_point {
+        let object = repository
+            .revparse_single(&start)
+            .map_err(|err| format!("Failed to resolve start point '{}': {}", start, err))?;
+        walk.push(object.id())
+            .map_err(|err| err.message().to_string())?;
+    } else {
+        walk.push_glob("*")
+            .map_err(|err| err.message().to_string())?;
+    }
+    Ok(())
+}
+
 /// Walks through the commits and adds each commit's Oid to the children of its parents.
 fn assign_children(commits: &mut [CommitInfo], indices: &HashMap<Oid, usize>) {
     for idx in 0..commits.len() {

src/main.rs

@@ -49,6 +49,7 @@ fn from_args() -> Result<(), String> {
         ses.settings.as_ref().unwrap(),
         ses.svg,
         ses.commit_limit,
+        ses.refspecs,
     )
 }
 
@@ -126,6 +127,14 @@ fn match_args() -> ArgMatches {
                 )
                 .required(false)
                 .num_args(1),
+        )
+        .arg(
+            Arg::new("refspecs")
+                .help(
+                    "Branch names or refspecs to show.\n  \
+                         Only the subgraph between the merge-base and the tips is displayed.",
+                )
+                .num_args(1..),
         );
 
     // Return match of declared arguments with what is present on command line
@@ -190,6 +199,11 @@ fn configure_session(ses: &mut Session, matches: &ArgMatches) -> Result<bool, St
     };
     ses.settings = Some(settings);
 
+    ses.refspecs = matches
+        .get_many::<String>("refspecs")
+        .map(|vals| vals.cloned().collect())
+        .unwrap_or_default();
+
     Ok(run_application)
 }
 
@@ -204,6 +218,7 @@ struct Session {
     pub repository: Option<Repository>,
     pub svg: bool,
     pub commit_limit: Option<usize>,
+    pub refspecs: Vec<String>,
 }
 
 impl Session {
@@ -219,6 +234,7 @@ impl Session {
             repository: None,
             svg: false,
             commit_limit: None,
+            refspecs: Vec::new(),
         }
     }
 }
@@ -597,9 +613,10 @@ fn run(
     settings: &Settings,
     svg: bool,
     max_commits: Option<usize>,
+    refspecs: Vec<String>,
 ) -> Result<(), String> {
     let now = Instant::now();
-    let graph = GitGraph::new(repository, settings, None, max_commits)?;
+    let graph = GitGraph::new(repository, settings, None, max_commits, refspecs)?;
 
     let duration_graph = now.elapsed().as_micros();