ref-filter: modify the 'lstrip=<N>' option to work with negative '<N>'

ref-filter: modify the 'lstrip=<N>' option to work with negative '<N>'

Currently the 'lstrip=<N>' option only takes a positive value '<N>'
and strips '<N>' slash-separated path components from the left. Modify
the 'lstrip' option to also take a negative number '<N>' which would
strip from the left as necessary and _leave_ behind only 'N'
slash-separated path components from the right-most end.

For e.g. %(refname:lstrip=-1) would make 'foo/goo/abc' into 'abc'.

Add documentation and tests for the same.

Signed-off-by: Karthik Nayak <Karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>