difftool: Handle finding mergetools/ in a path with spaces

difftool: Handle finding mergetools/ in a path with spaces

Use the original File::Find implementation from bf73fc2 (difftool:
print list of valid tools with '--tool-help', 2012-03-29) so that we
properly handle mergetools/ being located in a path containing
spaces.

One small difference is that we avoid using a global variable by
passing a reference to the list of tools.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>