[WIP][GitRepository] Implement #git_transaction

All actions git write actions implement `with_worktree`, so this is a
simple place to implement `git_transaction` and apply it safely.

TODO:  `GitRepository#worktree` probably doesn't need `#git_transaction`
since it should only be called from `#with_worktree`, but want to
confirm with others.  This commit will be rebased once that is
determined.

Author: NickLaMuro

app/models/git_repository.rb

@@ -71,9 +71,14 @@ def self_signed_cert_cb(_valid, _host)
     true
   end
 
+  # NOTE:  Please make sure to use this whenever you implement a method that is
+  # modifying the `.git` data (clone, fetch, checking, etc.) so that we can be
+  # sure this happens safely between processes.
   def with_worktree
-    handling_worktree_errors do
-      yield worktree
+    git_transaction do
+      handling_worktree_errors do
+        yield worktree
+      end
     end
   end
 
@@ -149,8 +154,10 @@ def refresh_tags
 
   def worktree
     @worktree ||= begin
-      clone_repo unless Dir.exist?(directory_name)
-      fetch_worktree
+      git_transaction do
+        clone_repo unless Dir.exist?(directory_name)
+        fetch_worktree
+      end
     end
   end