commit-graph: report incomplete chains during verification

commit-graph: report incomplete chains during verification

The load_commit_graph_chain_fd_st() function will stop loading chains
when it sees an error. But if it has loaded any graph slice at all, it
will return it. This is a good thing for normal use (we use what data we
can, and this is just an optimization). But it's a bad thing for
"commit-graph verify", which should be careful about finding any
irregularities. We do complain to stderr with a warning(), but the
verify command still exits with a successful return code.

The new tests here cover corruption of both the base and tip slices of
the chain. The corruption of the base file already works (it is the
first file we look at, so when we see the error we return NULL). The
"tip" case is what is fixed by this patch (it complains to stderr but
still returns the base slice).

Likewise the existing tests for corruption of the commit-graph-chain
file itself need to be updated. We already exited non-zero correctly for
the "base" case, but the "tip" case can now do so, too.

Note that this also causes us to adjust a test later in the file that
similarly corrupts a tip (though confusingly the test script calls this
"base"). It checks stderr but erroneously expects the whole "verify"
command to exit with a successful code.

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