feat(repo): Add AmendFastOptions::FromCommit

Existing `AmendFastOptions` only support amending from the working copy
or index, but we can't use either of these for implementing something
like squashing fixup commits because we need to use the entries from an
existing commit.

Author: claytonrcarter

git-branchless-lib/src/git/repo.rs

@@ -427,7 +427,7 @@ pub enum CreateCommitFastError {
 
 /// Options for `Repo::amend_fast`
 #[derive(Debug)]
-pub enum AmendFastOptions {
+pub enum AmendFastOptions<'repo> {
     /// Amend a set of paths from the current state of the working copy.
     FromWorkingCopy {
         /// The status entries for the files to amend.
@@ -438,14 +438,20 @@ pub enum AmendFastOptions {
         /// The paths to amend.
         paths: Vec<PathBuf>,
     },
+    /// Amend a set of paths from a different commit.
+    FromCommit {
+        /// The commit whose contents will be amended.
+        commit: Commit<'repo>,
+    },
 }
 
-impl AmendFastOptions {
+impl<'repo> AmendFastOptions<'repo> {
     /// Returns whether there are any paths to be amended.
     pub fn is_empty(&self) -> bool {
         match &self {
             AmendFastOptions::FromIndex { paths } => paths.is_empty(),
             AmendFastOptions::FromWorkingCopy { status_entries } => status_entries.is_empty(),
+            AmendFastOptions::FromCommit { commit } => commit.is_empty(),
         }
     }
 }
@@ -1422,10 +1428,14 @@ impl Repo {
     /// See `Repo::cherry_pick_fast` for motivation for performing the operation
     /// in-memory.
     #[instrument]
-    pub fn amend_fast(&self, parent_commit: &Commit, opts: &AmendFastOptions) -> Result<Tree> {
+    pub fn amend_fast(
+        &self,
+        parent_commit: &Commit,
+        opts: &AmendFastOptions,
+    ) -> std::result::Result<Tree, CreateCommitFastError> {
         let parent_commit_pathbufs = self
             .get_paths_touched_by_commit(parent_commit)?
-            .ok_or_else(|| Error::GetPatch {
+            .ok_or_else(|| CreateCommitFastError::GetPatch {
                 commit: parent_commit.get_oid(),
             })?
             .into_iter()
@@ -1439,6 +1449,11 @@ impl Repo {
                         result.extend(entry.paths().iter().cloned());
                     }
                 }
+                AmendFastOptions::FromCommit { commit } => {
+                    if let Some(paths) = self.get_paths_touched_by_commit(commit)? {
+                        result.extend(paths.iter().cloned());
+                    }
+                }
             };
             result.into_iter().collect_vec()
         };
@@ -1492,6 +1507,25 @@ impl Repo {
                     })
                     .collect::<HashMap<_, _>>()
             }
+            AmendFastOptions::FromCommit { commit } => {
+                let amended_tree = self.cherry_pick_fast(
+                    commit,
+                    parent_commit,
+                    &CherryPickFastOptions {
+                        reuse_parent_tree_if_possible: false,
+                    },
+                )?;
+                self.get_paths_touched_by_commit(commit)?
+                    .unwrap_or_default()
+                    .iter()
+                    .filter_map(|path| match amended_tree.get_path(path) {
+                        Ok(Some(entry)) => {
+                            Some((path.clone(), Some((entry.get_oid(), entry.get_filemode()))))
+                        }
+                        Ok(None) | Err(_) => None,
+                    })
+                    .collect::<HashMap<_, _>>()
+            }
         };
 
         // Merge the new path entries into the existing set of parent tree.

git-branchless/src/commands/amend.rs

@@ -373,6 +373,9 @@ pub fn amend(
                 "Amended with {uncommitted_changes}.",
             )?;
         }
+        AmendFastOptions::FromCommit { .. } => {
+            unreachable!("BUG: AmendFastOptions::FromCommit should not have been constructed.")
+        }
     }
 
     Ok(Ok(()))