* Optimize pulls merging
By utilizing `git clone -s --no-checkout` rather than cloning the whole
repo.
* Use sparse-checkout to speedup pulls merge
* Use bytes.Buffer instead of strings.Builder for backward compatibility
* Fix empty diff-tree output for repos with only the initial commit
* Fix missing argument for the format string
* Rework diff-tree-list generation
* Remove logging code
* File list for sparse-checkout must be prefix with /
Otherwise, they would match all files with the same name under
subdirectories.
* Update onto the rebased head
* Use referecen repo to avoid fetching objects
Mura Li