grep -P: add tests for matching ^ and $

Earlier, fba4f1 (grep -P: Fix matching ^ and $) fixed an ancient bug.  Add
some tests to protect the change from future breakages; a slightly broken
version of this was a part of the originally submitted patch.

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7810-grep.sh

@@ -47,6 +47,13 @@ test_expect_success setup '
 	echo vvv >t/v &&
 	mkdir t/a &&
 	echo vvv >t/a/v &&
+	{
+		echo "line without leading space1"
+		echo " line with leading space1"
+		echo " line with leading space2"
+		echo " line with leading space3"
+		echo "line without leading space2"
+	} >space &&
 	git add . &&
 	test_tick &&
 	git commit -m initial
@@ -893,4 +900,20 @@ test_expect_success 'mimic ack-grep --group' '
 	test_cmp expected actual
 '
 
+cat >expected <<EOF
+space: line with leading space1
+space: line with leading space2
+space: line with leading space3
+EOF
+
+test_expect_success LIBPCRE 'grep -E "^ "' '
+	git grep -E "^ " space >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success LIBPCRE 'grep -P "^ "' '
+	git grep -P "^ " space >actual &&
+	test_cmp expected actual
+'
+
 test_done