ref-filter: resurrect "strip" as a synonym to "lstrip"

ref-filter: resurrect "strip" as a synonym to "lstrip"

We forgot that "strip" was introduced at 0571979bd6 ("tag: do not
show ambiguous tag names as "tags/foo"", 2016-01-25) as part of Git
2.8 (and 2.7.1) when we started calling this "lstrip" to make it
easier to explain the new "rstrip" operation.

We shouldn't have renamed the existing one; "lstrip" should have
been a new synonym that means the same thing as "strip".  Scripts
in the wild are surely using the original form already.

Signed-off-by: Junio C Hamano <gitster@pobox.com>