commit-graph: fix segfault on e.g. "git status"

commit-graph: fix segfault on e.g. "git status"

When core.commitGraph=true is set, various common commands now consult
the commit graph. Because the commit-graph code is very trusting of
its input data, it's possibly to construct a graph that'll cause an
immediate segfault on e.g. "status" (and e.g. "log", "blame", ...). In
some other cases where git immediately exits with a cryptic error
about the graph being broken.

The root cause of this is that while the "commit-graph verify"
sub-command exhaustively verifies the graph, other users of the graph
simply trust the graph, and will e.g. deference data found at certain
offsets as pointers, causing segfaults.

This change does the bare minimum to ensure that we don't segfault in
the common fill_commit_in_graph() codepath called by
e.g. setup_revisions(), to do this instrument the "commit-graph
verify" tests to always check if "status" would subsequently
segfault. This fixes the following tests which would previously
segfault:

    not ok 50 - detect low chunk count
    not ok 51 - detect missing OID fanout chunk
    not ok 52 - detect missing OID lookup chunk
    not ok 53 - detect missing commit data chunk

Those happened because with the commit-graph enabled setup_revisions()
would eventually call fill_commit_in_graph(), where e.g.
g->chunk_commit_data is used early as an offset (and will be
0x0). With this change we get far enough to detect that the graph is
broken, and show an error instead. E.g.:

    $ git status; echo $?
    error: commit-graph is missing the Commit Data chunk
    1

That also sucks, we should *warn* and not hard-fail "status" just
because the commit-graph is corrupt, but fixing is left to a follow-up
change.

A side-effect of changing the reporting from graph_report() to error()
is that we now have an "error: " prefix for these even for
"commit-graph verify". Pseudo-diff before/after:

    $ git commit-graph verify
    -commit-graph is missing the Commit Data chunk
    +error: commit-graph is missing the Commit Data chunk

Changing that is OK. Various errors it emits now early on are prefixed
with "error: ", moving these over and changing the output doesn't
break anything.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>