completion: fix completing merge strategies on non-C locales

The anchor string "Available strategies are:" is translatable so
__git_list_merge_strategies may fail to collect available strategies
from 'git merge' on non-C locales. Force C locale on this command.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -594,7 +594,7 @@ __git_is_configured_remote ()
 
 __git_list_merge_strategies ()
 {
-	git merge -s help 2>&1 |
+	LANG=C LC_ALL=C git merge -s help 2>&1 |
 	sed -n -e '/[Aa]vailable strategies are: /,/^$/{
 		s/\.$//
 		s/.*://