grep: read patterns from stdin with -f -

Support the well-know convention of reading standard input instead of a
named file if "-" (dash) is specified.  GNU grep does the same.

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

t/t7810-grep.sh

@@ -303,6 +303,11 @@ test_expect_success 'grep -f, ignore empty lines' '
 	test_cmp expected actual
 '
 
+test_expect_success 'grep -f, ignore empty lines, read patterns from stdin' '
+	git grep -f - <patterns >actual &&
+	test_cmp expected actual
+'
+
 cat >expected <<EOF
 y:y yy
 --