userdiff: remove empty subexpression from elixir regex

userdiff: remove empty subexpression from elixir regex

The regex failed to compile on FreeBSD.

Also add /* -- */ mark to separate the two regex entries given to
the PATTERNS() macro, to make it consistent with patterns for other
content types.

Signed-off-by: Ed Maste <redacted>
Reviewed-by: Jeff King <redacted>
Helped-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>