add--interactive: quote commentChar regex

add--interactive: quote commentChar regex

Since c9d961647 (i18n: add--interactive: mark
edit_hunk_manually message for translation, 2016-12-14),
when the user asks to edit a hunk manually, we respect
core.commentChar in generating the edit instructions.
However, when we then strip out comment lines, we use a
simple regex like:

  /^$commentChar/

If your chosen comment character is a regex metacharacter,
then that will behave in a confusing manner ("$", for
instance, would only eliminate blank lines, not actual
comment lines).

We can fix that by telling perl not to respect
metacharacters.

Reported-by: Christian Rösch <christian@croesch.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>