am: invoke perl's strftime in C locale

We used to convert timestamps in metadata comment of Hg patch to
mbox-looking Date: field using strftime, without making sure the
resulting string is not translated.  Always use C locale for this.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ldv-alt

git-am.sh

@@ -261,7 +261,7 @@ split_patches () {
 			# Since we cannot guarantee that the commit message is in
 			# git-friendly format, we put no Subject: line and just consume
 			# all of the message as the body
-			perl -M'POSIX qw(strftime)' -ne 'BEGIN { $subject = 0 }
+			LANG=C LC_ALL=C perl -M'POSIX qw(strftime)' -ne 'BEGIN { $subject = 0 }
 				if ($subject) { print ; }
 				elsif (/^\# User /) { s/\# User/From:/ ; print ; }
 				elsif (/^\# Date /) {