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tail: fix follow for empty files (thanks cinap_lenrek)

--- a/sys/src/cmd/tail.c

+++ b/sys/src/cmd/tail.c

@@ -8,6 +8,7 @@

  * the simple command tail -c, legal in v10, is illegal

  */

+vlong	fend;

 long	count;

 int	anycount;

 int	follow;

@@ -111,7 +112,7 @@

 	else

 	if(units==LINES && origin==BEG)

 		skip();

-	if(follow && seekable)

+	if(follow && (seekable || fend == 0))

 		for(;;) {

 			static Dir *sb0, *sb1;

 			trunc(sb1, &sb0);

@@ -368,5 +369,6 @@

 static int

 isseekable(int fd)

 {

-	return seek(fd, 0, 2) > 0 && seek(fd, 0, 0) == 0;

+	fend = seek(fd, 0, 2);

+	return fend > 0 && seek(fd, 0, 0) == 0;

 }