supergit: implementing simple file-history search

This is done by stepping through a branch iterator to diff commits to
see where they changed.  This is somewhat computationally intensive,
and also doesn't work well yet because iterator stepping is done
internally and there is no API to extend iterators before they are
run.  That means that histories can only be read from first-parent
iterators.

Ideally we would do two things here:

1. build an API to let iterators extend themselves, either breadth
   first, or depth first.  But maybe that would be too much
   complexity for the iterator module?

2. figure out a better way to get the history of a file.  At the
   moment we are stepping through commits and diffing them with
   parents to find the set of changed paths.  I don't _think_ there is
   a way to simply compare refs, but maybe there is.

1 files changed, 22 insertions, 1 deletions

diff --git a/apps/servers/octopus/supergit/src/files.rs b/apps/servers/octopus/supergit/src/files.rs
index fa68fbc2f3dc..c593a4923921 100644
--- a/apps/servers/octopus/supergit/src/files.rs
+++ b/apps/servers/octopus/supergit/src/files.rs
@@ -1,7 +1,7 @@
 use crate::{Branch, BranchIter, Commit, HashId};
 use atomptr::AtomPtr;
 use git2::{ObjectType, TreeWalkMode, TreeWalkResult};
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, BTreeSet};
 use std::{path::PathBuf, sync::Arc};
 
 /// A tree of files
@@ -76,6 +76,27 @@ impl FileTree {
     pub fn load(&self, path: &str) -> Option<Yield> {
         self.get_entry(path).and_then(|e| e.load(&self.repo))
     }
+
+    /// Get the history of a path with a branch iterator
+    ///
+    /// This function is very computationally intensive, because it
+    /// will step through the entire iterator to pull commits from,
+    /// and see if they touch the respective path.
+    pub fn history(&self, iter: BranchIter, path: &str) -> Vec<Commit> {
+        iter.filter_map(|c| {
+            if c.commit()
+                .get_paths()
+                .into_iter()
+                .collect::<BTreeSet<_>>()
+                .contains(path)
+            {
+                Some(c.commit().clone())
+            } else {
+                None
+            }
+        })
+        .collect()
+    }
 }
 
 /// Data yielded from loading a part of the file tree