Let core.excludesfile default to $XDG_CONFIG_HOME/git/ignore

Let core.excludesfile default to $XDG_CONFIG_HOME/git/ignore

To use the feature of core.excludesfile, the user needs:

 1. to create such a file,

 2. and add configuration variable to point at it.

Instead, we can make this a one-step process by choosing a default value
which points to a filename in the user's $HOME, that is unlikely to
already exist on the system, and only use the presence of the file as a
cue that the user wants to use that feature.

And we use "${XDG_CONFIG_HOME:-$HOME/.config/git}/ignore" as such a
file, in the same directory as the newly added configuration file
("${XDG_CONFIG_HOME:-$HOME/.config/git}/config).  The use of this
directory is in line with XDG specification as a location to store
such application specific files.

Signed-off-by: Huynh Khoi Nguyen Nguyen <Huynh-Khoi-Nguyen.Nguyen@ensimag.imag.fr>
Signed-off-by: Valentin Duperray <Valentin.Duperray@ensimag.imag.fr>
Signed-off-by: Franck Jonas <Franck.Jonas@ensimag.imag.fr>
Signed-off-by: Lucien Kong <Lucien.Kong@ensimag.imag.fr>
Signed-off-by: Thomas Nguy <Thomas.Nguy@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>