Revert "Run git config instead of reading config file manually (#639

)"

This reverts commit e8fe2bd.

src/resolvers/git.cr

@@ -351,7 +351,10 @@ module Shards
     end
 
     private def valid_repository?
-      capture("git config --get-regexp 'remote\\..+\\.mirror'").each_line.any?(&.==("true"))
+      File.each_line(File.join(local_path, "config")) do |line|
+        return true if line =~ /mirror\s*=\s*true/
+      end
+      false
     end
 
     private def origin_url