Document ls-files -t as semi-obsolete.

Document ls-files -t as semi-obsolete.

The behavior of "git ls-files -t" is very misleading (see
http://thread.gmane.org/gmane.comp.version-control.git/126516 and
http://thread.gmane.org/gmane.comp.version-control.git/144394/focus=144397
for examples of mislead users) and badly documented, hence we point the
users to superior alternatives.

The feature is marked as "semi-obsolete" but not "scheduled for removal"
since it's a plumbing command, scripts might use it, and Git testsuite
already uses it to test the state of the index.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>