Project.pm: optimize project config file loading
Instead of running `git config` once for each config file
setting to be loaded, read all the local project config
file settings with a single `git config` call and access
them from the hash generated from that output.
This reduces the number of `git config` callouts from
12 to 1 when loading a project and makes it possible
to add loading of more config options in the future
without increasing the number of `git config` callouts.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>