Merge branch 'mm/config-edit-global'

Start "git config --edit --global" from a skeletal per-user
configuration file contents, instead of a total blank, when the
user does not already have any.  This immediately reduces the need
for a later "Have you forgotten setting core.user?" and we can add
more to the template as we gain more experience.

* mm/config-edit-global:
commit: advertise config --global --edit on guessed identity
home_config_paths(): let the caller ignore xdg path
config --global --edit: create a template file if needed

diff --cc builtin/commit.c
Simple merge

diff --cc builtin/config.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc path.c
Simple merge