sync_default_gems.rb: do not reset the whole directory [ci skip]

If no files to be ignored, resetting with no argument means
resetting the whole directory.

Author: nobu

tool/sync_default_gems.rb

@@ -439,7 +439,7 @@ def sync_default_gems_with_commits(gem, ranges, edit: nil)
         system(*%W"git reset HEAD --", *ignore)
         File.unlink(*ignore)
         ignore = IO.popen(%W"git status --porcelain" + ignore, &:readlines).map! {|line| line[/^.. (.*)/, 1]}
-        system(*%W"git checkout HEAD --", *ignore)
+        system(*%W"git checkout HEAD --", *ignore) unless ignore.empty?
       end
       unless conflict.empty?
         if edit