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>

git-add--interactive.perl

@@ -1097,7 +1097,7 @@ EOF2
 
 	open $fh, '<', $hunkfile
 		or die sprintf(__("failed to open hunk edit file for reading: %s"), $!);
-	my @newtext = grep { !/^$comment_line_char/ } <$fh>;
+	my @newtext = grep { !/^\Q$comment_line_char\E/ } <$fh>;
 	close $fh;
 	unlink $hunkfile;
 

t/t3701-add-interactive.sh

@@ -380,4 +380,12 @@ test_expect_success 'patch mode ignores unmerged entries' '
 	test_cmp expected diff
 '
 
+test_expect_success 'hunk-editing handles custom comment char' '
+	git reset --hard &&
+	echo change >>file &&
+	test_config core.commentChar "\$" &&
+	echo e | GIT_EDITOR=true git add -p &&
+	git diff --exit-code
+'
+
 test_done