Only do backend checks for push and pull commands

A lot of commands will work without a .gitfat file and so git-fat
shouldn't care whether or not it's there unless it's needed.
Remove noisy pylint option

push_files isn't required to be implemented, but pylint is complaining
that it's not implemented in the http backend
Merge pull request #24 from abraithwaite/fix-config

Only do backend checks for push and pull commands