spec: Do not recreate custom repositories

There are two project factories which create repositories, once with a
default repository and once with custom files which then get written
into the repository. These can be used in combination to use a default
repository which has a bunch of custom files on top, and some tests do
in fact do this. When doing so, we call Gitaly's `CreateRepository()`
RPC twice though, where the second call is idempotent and doesn't really
do anything interesting.

Gitaly is about to change behaviour of `CreateRepository()` to fail in
case it is asked to recreate an already existing repository. So let's
call `CreateRepository()` for custom repos only when they do not exist
yet.

spec/factories/projects.rb

@@ -190,7 +190,7 @@ FactoryBot.define do
       end
 
       after :create do |project, evaluator|
-        raise "Failed to create repository!" unless project.create_repository
+        raise "Failed to create repository!" unless project.repository.exists? || project.create_repository
 
         evaluator.files.each do |filename, content|
           project.repository.create_file(