config --global --edit: create a template file if needed

When the user has no ~/.gitconfig file, git config --global --edit used
to launch an editor on an nonexistant file name.

Instead, create a file with a default content before launching the
editor. The template contains only commented-out entries, to save a few
keystrokes for the user. If the values are guessed properly, the user
will only have to uncomment the entries.

Advanced users teaching newbies can create a minimalistic configuration
faster for newbies. Beginners reading a tutorial advising to run "git
config --global --edit" as a first step will be slightly more guided for
their first contact with Git.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
home_config_paths(): let the caller ignore xdg path

The caller can signal that it is not interested in learning
the location of $HOME/.gitconfig by passing global=NULL, but
there is no way to decline the path to the configuration
file based on $XDG_CONFIG_HOME.

Allow the caller to pass xdg=NULL to signal that it is not
interested in the XDG location.

Commit-message-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
commit: advertise config --global --edit on guessed identity

When the user has no user-wide configuration file, it's faster to use the
newly introduced config file template than to run two commands to set
user.name and user.email. Advise this to the user.

The old advice is kept if the user already has a configuration file since
the template feature would not trigger in this case.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'mm/config-edit-global' into jch

* 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
t9814: fix misconversion from test $a -o $b to test $a || test $b

Spotted-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'ep/avoid-test-a-o' into jch

* ep/avoid-test-a-o:
  t9814: fix misconversion from test $a -o $b to test $a || test $b