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, 28 insertions, 1 deletions

diff --git a/apps/servers/octopus/supergit/src/diff.rs b/apps/servers/octopus/supergit/src/diff.rs
index e92a4cd1804c..e83903cf6ebc 100644
--- a/apps/servers/octopus/supergit/src/diff.rs
+++ b/apps/servers/octopus/supergit/src/diff.rs
@@ -1,5 +1,32 @@
-
 /// A diff between two commits
+///
+/// At the moment this type doesn't properly express a Diff, and is
+/// only used to compute the change set between commits to generate a
+/// file history.
 pub struct Diff {
+    paths: Vec<String>,
+}
+
+impl Diff {
+    /// Generate a new Diff from a git2::Diff
+    pub(crate) fn from(d: git2::Diff) -> Self {
+        Self {
+            paths: d.deltas().fold(vec![], |mut vec, delta| {
+                append(&mut vec, delta.old_file());
+                append(&mut vec, delta.new_file());
+                vec
+            }),
+        }
+    }
+
+    /// Get all paths touched by a diff
+    pub fn get_paths(&self) -> Vec<String> {
+        self.paths.clone()
+    }
+}
 
+fn append(vec: &mut Vec<String>, f: git2::DiffFile) {
+    if let Some(path) = f.path().map(|p| p.to_str().unwrap().into()) {
+        vec.push(path);
+    }
 }