ignore: info/exclude should trump core.excludesfile

ignore: info/exclude should trump core.excludesfile

$GIT_DIR/info/exclude and core.excludesfile (which falls back to
$XDG_HOME/git/ignore) are both ways to override the ignore pattern
lists given by the project in .gitignore files.  The former, which
is per-repository personal preference, should take precedence over
the latter, which is a personal preference default across different
repositories that are accessed from that machine.  The existing
documentation also agrees.

However, the precedence order was screwed up between these two from
the very beginning when 896bdfa2 (add: Support specifying an
excludes file with a configuration variable, 2007-02-27) introduced
core.excludesfile variable.

Noticed-by: Yohei Endo <yoheie@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>