Merge branch 'mm/config-xdg'

Teach git to read various information from $XDG_CONFIG_HOME/git/ to allow
the user to avoid cluttering $HOME.

* mm/config-xdg:
config: write to $XDG_CONFIG_HOME/git/config file when appropriate
Let core.attributesfile default to $XDG_CONFIG_HOME/git/attributes
Let core.excludesfile default to $XDG_CONFIG_HOME/git/ignore
config: read (but not write) from $XDG_CONFIG_HOME/git/config file

diff --cc Documentation/config.txt
Simple merge

diff --cc cache.h
Simple merge

diff --cc dir.c
Simple merge