ref-filter: use generation number for --contains

ref-filter: use generation number for --contains

A commit A can reach a commit B only if the generation number of A
is strictly larger than the generation number of B. This condition
allows significantly short-circuiting commit-graph walks.

Use generation number for '--contains' type queries.

On a copy of the Linux repository where HEAD is contained in v4.13
but no earlier tag, the command 'git tag --contains HEAD' had the
following peformance improvement:

Before: 0.81s
After:  0.04s
Rel %:  -95%

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>