Add ps::private::cherry_picking module

Add ps::private::cherry_picking module so that we have a place to house
Patch Stack specific cherry picking concerns. To start we have added the
map_range_for_cherry_pick() function that translates patch indexs to
struct ready to be used with the git modules cherry pick functions.

<!-- ps-id: c0921e7b-2d0c-429a-86fa-bac397a665c3 -->

src/ps/private/cherry_picking.rs

@@ -0,0 +1,51 @@
+use super::super::super::ps;
+use std::result::Result;
+
+pub struct CherryPickRange {
+    pub root_oid: git2::Oid,
+    pub leaf_oid: Option<git2::Oid>,
+}
+
+#[derive(Debug)]
+pub enum MapRangeForCherryPickError {
+    StartIndexNotFound,
+    EndIndexNotFound,
+}
+
+pub fn map_range_for_cherry_pick(
+    patches_vec: &Vec<ps::ListPatch>,
+    start_patch_index: usize,
+    end_patch_index: Option<usize>,
+) -> Result<CherryPickRange, MapRangeForCherryPickError> {
+    let start_patch_oid = patches_vec
+        .get(start_patch_index)
+        .ok_or(MapRangeForCherryPickError::StartIndexNotFound)?
+        .oid;
+
+    Ok(match (start_patch_index, end_patch_index) {
+        (si, Some(ei)) if (si < ei) => {
+            let end_patch_oid = patches_vec
+                .get(ei)
+                .ok_or(MapRangeForCherryPickError::EndIndexNotFound)?
+                .oid;
+            CherryPickRange {
+                root_oid: start_patch_oid,
+                leaf_oid: Some(end_patch_oid),
+            }
+        }
+        (si, Some(ei)) if (si > ei) => {
+            let end_patch_oid = patches_vec
+                .get(ei)
+                .ok_or(MapRangeForCherryPickError::EndIndexNotFound)?
+                .oid;
+            CherryPickRange {
+                root_oid: end_patch_oid,
+                leaf_oid: Some(start_patch_oid),
+            }
+        }
+        _ => CherryPickRange {
+            root_oid: start_patch_oid,
+            leaf_oid: None,
+        },
+    })
+}

src/ps/private/mod.rs

@@ -1,4 +1,5 @@
 pub mod checkout;
+pub mod cherry_picking;
 pub mod commit_is_behind;
 pub mod config;
 pub mod git;