tail compat: fix header printing for tail -f

Makes tail -f for multiple files display the header only if another (ie. not
currently displayed) file changed.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

TODO

@@ -127,11 +127,6 @@ patch
 
   And while we're at it, a new patch filename quoting format is apparently
   coming soon:  http://marc.theaimsgroup.com/?l=git&m=112927316408690&w=2
----
-tail
-  ./busybox tail -f foo.c~ TODO
-  should not print fmt=header_fmt for subsequent date >> TODO; i.e. only
-  fmt+ if another (not the current) file did change
 
 Architectural issues: