check_everything_connected: assume alternate ref tips are valid

check_everything_connected: assume alternate ref tips are valid

When we receive a remote ref update to sha1 "X", we want to check that
we have all of the objects needed by "X". We can assume that our
repository is not currently corrupted, and therefore if we have a ref
pointing at "Y", we have all of its objects. So we can stop our
traversal from "X" as soon as we hit "Y".

If we make the same non-corruption assumption about any repositories we
use to store alternates, then we can also use their ref tips to shorten
the traversal.

This is especially useful when cloning with "--reference", as we
otherwise do not have any local refs to check against, and have to
traverse the whole history, even though the other side may have sent us
few or no objects. Here are results for the included perf test (which
shows off more or less the maximal savings, getting one new commit and
sharing the whole history):

Test                        HEAD^             HEAD
--------------------------------------------------------------------
[on git.git]
5600.3: clone --reference   2.94(2.86+0.08)   0.09(0.08+0.01) -96.9%
[on linux.git]
5600.3: clone --reference   45.74(45.34+0.41)   0.36(0.30+0.08) -99.2%

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>