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, 3 insertions, 2 deletions

diff --git a/editors/diff.c b/editors/diff.c
index eac4b08c5..4e51f6f76 100644
--- a/editors/diff.c
+++ b/editors/diff.c
@@ -1021,11 +1021,12 @@ static unsigned diffreg(char *file1, char *file2, int flags)
         rval = D_SAME;
 
         if (flags & D_EMPTY1)
-                f1 = xfopen(bb_dev_null, "r");
+                /* can't be stdin, but xfopen_stdin() is smaller code */
+                f1 = xfopen_stdin(bb_dev_null);
         else
                 f1 = xfopen_stdin(file1);
         if (flags & D_EMPTY2)
-                f2 = xfopen(bb_dev_null, "r");
+                f2 = xfopen_stdin(bb_dev_null);
         else
                 f2 = xfopen_stdin(file2);