mergetool-lib: fix default tool selection

mergetool-lib: fix default tool selection

When no diff nor merge tool is specified (config, option), mergetool-lib
is supposed to choose a default tool from a set of tools. That set is
constructed dynamically depending on the environment (graphical, editor
setting) as a space separated string of tool names.

719518f (mergetool--lib: set IFS for difftool and mergetool, 2015-05-20)
introduced a newline as IFS which breaks the parsing of the space
separated list into items, resulting in a failed search for an available
tool.

Set IFS to a space locally for the tool search.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>