This cleans up a number of items suggested during code review
with @vmg, including:

* renaming "outside repo" config API to `git_config_open_default`
* killing the `git_config_open_global` API
* removing the `git_` prefix from the static functions in fileops
* removing some unnecessary functionality from the "cp" command

This extends git_repository_init_ext further with support for
initializing the repository from an external template directory
and with support for the "create shared" type flags that make a
set GID repository directory.

This also adds tests for much of the new functionality to the
existing `repo/init.c` test suite.

Also, this adds a bunch of new utility functions including a
very general purpose `git_futils_mkdir` (with the ability to
make paths and to chmod the paths post-creation) and a file
tree copying function `git_futils_cp_r`.  Also, this includes
some new path functions that were useful to keep the code
simple.