bash completion: complete variable names for "git config" with options

bash completion: complete variable names for "git config" with options

This makes it easier for users to get and unset their configuration
variables without having to open documentation or dig through their
configuration file.

__git_config_get_set_variables() retrieves the set configuration
variables from the appropriate configuration file. For example, if
the user has previously specified --global only the global variables
are returned. The same applies for --system, and --file. If no
location has been specified, all set variables are returned.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>