when converting sha1 hash to filename, use first 2 charactors as fold…

[qigtang]

…er name for better performance

I am using git-pat to manage train data set (millions of files). Need to change file name to folder/filename to avoid stressing the OS file system out.

[jedbrown]

I think this is a good change and would be happy to merge if you can make it maintain backward compatibility. We'd either need to notice if the store uses the old schema and have a way to upgrade it or be able to work with a mixed-format store. Also, we can't just rip out "v1" parts without an upgrade path.

[qigtang]

Just uploaded the python script I used to update storage name. Sorry I have no way to test v1/v2 compatibility, so I wiped out v1.