Fix: Git resolver doesn't pull tags anymore

Creating bare Git repositories doesn't configure the local
repository to fetch remote refs, such as branches and tags, which
totally broke the update command.

This patch reverts back to mirroring Git repositories, and verifies
whether a cloned repository is valid, or not before fetching new
refs.

fixes #211

src/resolvers/git.cr

@@ -181,16 +181,23 @@ module Shards
       Shards.logger.info "Fetching #{git_url}"
 
       if cloned_repository?
-        fetch_repository
+        # repositories cloned with shards v0.8.0 won't fetch any new remote
+        # refs; we must delete them and clone again!
+        if valid_repository?
+          fetch_repository
+        else
+          delete_repository
+          mirror_repository
+        end
       else
-        clone_repository
+        mirror_repository
       end
 
       @updated_cache = true
     end
 
-    private def clone_repository
-      run_in_current_folder "git clone --bare --quiet -- #{FileUtils.escape git_url} #{local_path}"
+    private def mirror_repository
+      run_in_current_folder "git clone --mirror --quiet -- #{FileUtils.escape git_url} #{local_path}"
     rescue Error
       raise Error.new("Failed to clone #{git_url}")
     end
@@ -210,6 +217,13 @@ module Shards
       Dir.exists?(local_path)
     end
 
+    private def valid_repository?
+      File.each_line(File.join(local_path, "config")) do |line|
+        return true if line =~ /mirror\s*=\s*true/
+      end
+      false
+    end
+
     private def origin_changed?
       (@origin_url ||= capture("git ls-remote --get-url origin").strip) != git_url
     end