improve performance of commit-graph variant ever-so-slightly

In practice, what really matters is to reuse as much of the
previous computation as possible.

Re-using the graph (or its menory) doesn't do much, but reusing
the parsed commits would be a huge benefit when running many
queries after another.

gix-revision/src/merge_base.rs

@@ -44,11 +44,12 @@ pub(crate) mod function {
  others: &[ObjectId],
  graph: &mut Graph<'_, '_, Flags>,
  ) -> Result<Option<Vec<ObjectId>>, Error> {
- let _span = gix_trace::coarse!("gix_revision::merge_base()", ?first, ?others,);
+ let _span = gix_trace::coarse!("gix_revision::merge_base()", ?first, ?others);
  if others.is_empty() || others.contains(&first) {
  return Ok(Some(vec![first]));
  }
 
+ graph.clear();
  let bases = paint_down_to_common(first, others, graph)?;
  graph.clear();
 
@@ -65,6 +66,7 @@ pub(crate) mod function {
  if commits.is_empty() {
  return Ok(Vec::new());
  }
+ let _span = gix_trace::detail!("gix_revision::remove_redundant()", num_commits = %commits.len());
  let sorted_commits = {
  let mut v = commits.to_vec();
  v.sort_by(|a, b| a.1.cmp(&b.1));
@@ -217,11 +219,10 @@ pub(crate) mod function {
  type Error = gix_object::decode::Error;
 
  fn try_from(commit: LazyCommit<'_, '_>) -> Result<Self, Self::Error> {
+ let (generation, timestamp) = commit.generation_and_timestamp()?;
  Ok(GenThenTime {
- generation: commit
- .generation()
- .unwrap_or(gix_commitgraph::GENERATION_NUMBER_INFINITY),
- time: commit.committer_timestamp()?,
+ generation: generation.unwrap_or(gix_commitgraph::GENERATION_NUMBER_INFINITY),
+ time: timestamp,
  })
  }
  }