grep: make "-f -" work (+ testsuite)

diff: small code shrink

function                                             old     new   delta
grep_main                                            722     714      -8
diffreg                                             1825    1793     -32
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-40)             Total: -40 bytes

1 files changed, 4 insertions, 0 deletions

diff --git a/testsuite/grep.tests b/testsuite/grep.tests
index 34c0fe4f7..b2de2af54 100755
--- a/testsuite/grep.tests
+++ b/testsuite/grep.tests
@@ -78,6 +78,10 @@ testing "grep handles multiple regexps" "grep -e one -e two input ; echo \$?" \
 testing "grep -F handles multiple expessions" "grep -F -e one -e two input ; echo \$?" \
 	"one\ntwo\n0\n" "one\ntwo\n" ""
 
+# -f file/-
+testing "grep can read regexps from stdin" "grep -f - input ; echo \$?" \
+	"two\nthree\n0\n" "tw\ntwo\nthree\n" "tw.\nthr\n"
+
 optional FEATURE_GREP_EGREP_ALIAS
 testing "grep -E supports extended regexps" "grep -E fo+" "foo\n" "" \
 	"b\ar\nfoo\nbaz"