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>

Author: Michael J Gruber

git-mergetool--lib.sh

@@ -305,6 +305,7 @@ guess_merge_tool () {
 	EOF
 
 	# Loop over each candidate and stop when a valid merge tool is found.
+	IFS=' '
 	for tool in $tools
 	do
 		is_available "$tool" && echo "$tool" && return 0