feat: Head::prior_checked_out_branches() (#427)

git-repository/src/head.rs

@@ -83,6 +83,8 @@ impl<'repo> Head<'repo> {
 }
 ///
 pub mod log {
+    use crate::bstr::{BString, ByteSlice};
+    use git_hash::ObjectId;
     use std::convert::TryInto;
 
     use crate::Head;
@@ -96,6 +98,22 @@ pub mod log {
                 buf: Vec::new(),
             }
         }
+
+        /// Return a list of all branch names that were previously checked out with the first-ever checked out branch
+        /// being the first entry of the list, and the most recent is the last, along with the commit they were pointing to
+        /// at the time.
+        pub fn prior_checked_out_branches(&self) -> std::io::Result<Option<Vec<(BString, ObjectId)>>> {
+            Ok(self.log_iter().all()?.map(|log| {
+                log.filter_map(Result::ok)
+                    .filter_map(|line| {
+                        line.message
+                            .strip_prefix(b"checkout: moving from ")
+                            .and_then(|from_to| from_to.find(" to ").map(|pos| &from_to[..pos]))
+                            .map(|from_branch| (from_branch.as_bstr().to_owned(), line.previous_oid()))
+                    })
+                    .collect()
+            }))
+        }
     }
 }