Code style updates in Server
Move reading commit message file into Repository

This eliminates the need to pass an argument to #commit and seems more
cohesive given that this is directly related to creating a commit.
It also opens up the possibility for moving the writing of the commit
message file into the Repository in the future.
Extract writing the commit message from Server

...and put it into the Repository.
* #file_revert handles writing its commit message and to account for a
  nil revision.
* #file_write will write out data to a file, ensure that the
  necessary path exists and write the commit message

Additionally, I extracted getting the absolute path for a file in the
repository into a private method.