Originally, the `git_config` struct is a collection of all the parsed
configuration files from different scopes (system-wide config,
user-specific config as well as the repo-specific config files).
Historically, we didn't and don't yet have any other configuration
backends than the one for files, which is why the field holding the
config backends is called `files`. But in fact, nothing dictates that
the vector of backends actually holds file backends only, as they are
generic and custom backends can be implemented by users.

Rename the member to be called `backends` to clarify that there is
nothing specific to files here.