transaction: move construction of MutableRepo out of Transaction::new()

I'm about to move `MutableRepo` to the `repo` module and it will make
more sense to have the construction of it there then.

lib/src/repo.rs

@@ -31,7 +31,7 @@ use crate::settings::{RepoSettings, UserSettings};
 use crate::store;
 use crate::store::{Store, StoreError};
 use crate::store_wrapper::StoreWrapper;
-use crate::transaction::Transaction;
+use crate::transaction::{MutableRepo, Transaction};
 use crate::view::{ReadonlyView, View};
 use crate::working_copy::WorkingCopy;
 
@@ -284,12 +284,8 @@ impl ReadonlyRepo {
     }
 
     pub fn start_transaction(&self, description: &str) -> Transaction {
-        Transaction::new(
-            &self,
-            &self.view,
-            &self.evolution.as_ref().unwrap(),
-            description,
-        )
+        let mut_repo = MutableRepo::new(self, &self.view, &self.evolution.as_ref().unwrap());
+        Transaction::new(mut_repo, description)
     }
 
     pub fn reload(&mut self) {

lib/src/transaction.rs

@@ -41,13 +41,7 @@ pub struct MutableRepo<'r> {
 }
 
 impl<'r> Transaction<'r> {
-    pub fn new(
-        repo: &'r ReadonlyRepo,
-        view: &ReadonlyView,
-        evolution: &ReadonlyEvolution<'r>,
-        description: &str,
-    ) -> Transaction<'r> {
-        let mut_repo = MutableRepo::new(repo, view, evolution);
+    pub fn new(mut_repo: Arc<MutableRepo<'r>>, description: &str) -> Transaction<'r> {
         Transaction {
             repo: Some(mut_repo),
             description: description.to_owned(),