credential-cache: use XDG_CACHE_HOME for socket

credential-cache: use XDG_CACHE_HOME for socket

Make git-credential-cache follow the XDG base path specification by
default. This increases consistency with other applications and helps
keep clutter out of users' home directories.

Check the old socket location, ~/.git-credential-cache/, and use
~/.git-credential-cache/socket if that directory exists rather than
forcing users who have used `git credential-cache` before to migrate to
the new XDG compliant location.
Otherwise use the socket $XDG_CACHE_HOME/git/credential/socket following
XDG base path specification. Use the subdirectory credential/ in case
other files are cached under $XDG_CACHE_HOME/git/ in the future and to
make the socket's purpose clear.

Signed-off-by: Devin Lehmacher <lehmacdj@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>