Add ext_common_ancestor feature to Cargo.toml

Add ext_common_ancestor feature to Cargo.toml so that we could
experiment with using the external common ancestor functionality rather
than the built in one to see if it is more performant when dealing with
larger numbers of branches.

<!-- ps-id: c1e664d4-101e-4817-9ea9-e5dc7a1df114 -->

Cargo.toml

@@ -41,3 +41,4 @@ clap_mangen = "0.2.14"
 
 [features]
 backup_cmd = []
+ext_common_ancestor = []

src/ps/private/git.rs

@@ -39,6 +39,7 @@ mod create_commit;
 mod create_cwd_repo;
 mod create_signed_commit;
 mod create_unsigned_commit;
+mod ext_common_ancestor;
 mod ext_delete_remote_branch;
 mod ext_fetch;
 mod ext_push;
@@ -68,6 +69,7 @@ pub use create_commit::*;
 pub use create_cwd_repo::*;
 pub use create_signed_commit::*;
 pub use create_unsigned_commit::*;
+pub use ext_common_ancestor::*;
 pub use ext_delete_remote_branch::*;
 pub use ext_fetch::*;
 pub use ext_push::*;

src/ps/private/git/ext_common_ancestor.rs

@@ -0,0 +1,53 @@
+use super::super::utils;
+use git2;
+use std::result::Result;
+
+#[derive(Debug)]
+pub enum ExtCommonAncestorError {
+    Unhandled(Box<dyn std::error::Error>),
+}
+
+impl std::fmt::Display for ExtCommonAncestorError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Unhandled(e) => write!(f, "{}", e),
+        }
+    }
+}
+
+impl std::error::Error for ExtCommonAncestorError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Self::Unhandled(e) => Some(e.as_ref()),
+        }
+    }
+}
+
+impl From<utils::ExecuteWithOutputError> for ExtCommonAncestorError {
+    fn from(value: utils::ExecuteWithOutputError) -> Self {
+        Self::Unhandled(value.into())
+    }
+}
+
+impl From<std::string::FromUtf8Error> for ExtCommonAncestorError {
+    fn from(value: std::string::FromUtf8Error) -> Self {
+        Self::Unhandled(value.into())
+    }
+}
+
+impl From<git2::Error> for ExtCommonAncestorError {
+    fn from(value: git2::Error) -> Self {
+        Self::Unhandled(value.into())
+    }
+}
+
+pub fn ext_common_ancestor(
+    one: git2::Oid,
+    two: git2::Oid,
+) -> Result<git2::Oid, ExtCommonAncestorError> {
+    let output =
+        utils::execute_with_output("git", &["merge-base", &one.to_string(), &two.to_string()])?;
+    let sha = String::from_utf8(output.stdout)?;
+    let common_ancestor_oid = git2::Oid::from_str(sha.trim())?;
+    Ok(common_ancestor_oid)
+}

src/ps/private/state_computation.rs

@@ -245,7 +245,7 @@ pub fn get_list_branch_info(
 #[derive(Debug)]
 pub enum GetPatchInfoCollectionError {
     GetBranchHeadOid,
-    GetCommonAncestor(git::CommonAncestorError),
+    GetCommonAncestor(Box<dyn std::error::Error>),
     GetCommits(git::GitError),
     GetRevisionOid(git2::Error),
     FindCommit(git2::Error),
@@ -269,7 +269,7 @@ impl std::error::Error for GetPatchInfoCollectionError {
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match self {
             Self::GetBranchHeadOid => None,
-            Self::GetCommonAncestor(e) => Some(e),
+            Self::GetCommonAncestor(e) => Some(e.as_ref()),
             Self::GetCommits(e) => Some(e),
             Self::GetRevisionOid(e) => Some(e),
             Self::FindCommit(e) => Some(e),
@@ -296,8 +296,13 @@ pub fn get_patch_info_collection(
         .get()
         .target()
         .ok_or(GetPatchInfoCollectionError::GetBranchHeadOid)?;
+
+    #[cfg(not(feature = "ext_common_ancestor"))]
     let common_ancestor_oid = git::common_ancestor(repo, branch_head_oid, base_oid)
-        .map_err(GetPatchInfoCollectionError::GetCommonAncestor)?;
+        .map_err(|e| GetPatchInfoCollectionError::GetCommonAncestor(e.into()))?;
+    #[cfg(feature = "ext_common_ancestor")]
+    let common_ancestor_oid = git::ext_common_ancestor(branch_head_oid, base_oid)
+        .map_err(|e| GetPatchInfoCollectionError::GetCommonAncestor(e.into()))?;
 
     let revwalk = git::get_revs(
         repo,