grep: use REG_STARTEND for all matching if available

Refactor REG_STARTEND handling inlook_ahead() into a new helper,
regmatch(), and use it for line matching, too.  This allows regex
matching beyond NUL characters if regexec() supports the flag.  NUL
characters themselves are not matched in any way, though.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7008-grep-binary.sh

@@ -59,4 +59,14 @@ test_expect_success 'git grep -Fi iLE a' '
 	git grep -Fi iLE a
 '
 
+# This test actually passes on platforms where regexec() supports the
+# flag REG_STARTEND.
+test_expect_failure 'git grep ile a' '
+	git grep ile a
+'
+
+test_expect_failure 'git grep .fi a' '
+	git grep .fi a
+'
+
 test_done