[BUGFIX] commit-graph: fix bug around octopus merges

[derrickstolee]

The commit message below describes the bug. This has been sent upstream
hopefully in time for v2.23.0. If we are taking 2.23.0 soon, then we would not
need this fix.

The question for @jrbriggs and others is: do we rush this fix to the version
of microsoft/vfsforgit in EA? There are possible reasons that this is an
edge-case enough to ignore:

1. It only affects the split commit-graph when a non-base file has an
   octopus merge commit. Only one octopus merge exists in the OS repo
   right now.

2. It will not break users, but will cause "warning: commit-graph chain
   does not match" warnings and cause increasing performance issues as
   the base commit-graph files far further and further behind.

I'm open to all options here.

Thanks,
-Stolee

---

In 1771be9 "commit-graph: merge commit-graph chains" (2019-06-18),
the method sort_and_scan_merged_commits() was added to merge the
commit lists of two commit-graph files in the incremental format.
Unfortunately, there was an off-by-one error in that method around
incrementing num_extra_edges, which leads to an incorrect offset
for the base graph chunk.

When we store an octopus merge in the commit-graph file, we store
the first parent in the normal place, but use the second parent
position to point into the "extra edges" chunk where the remaining
parents exist. This means we should be adding "num_parents - 1"
edges to this list, not "num_parents - 2". That is the basic error.

The reason this was not caught in the test suite is more subtle.
In 5324-split-commit-graph.sh, we test creating an octopus merge
and adding it to the tip of a commit-graph chain, then verify the
result. This should have caught the problem, except that when
we load the commit-graph files we were overly careful to not fail
when the commit-graph chain does not match. This care was on
purpose to avoid race conditions as one process reads the chain
and another process modifies it. In such a case, the reading
process outputs the following message to stderr:

warning: commit-graph chain does not match

These warnings are output in the test suite, but ignored. By
checking the stderr of git commit-graph verify to include
the expected progress output, it will now catch this error.

Signed-off-by: Derrick Stolee dstolee@microsoft.com

[jrbriggs]

Given the potential impact, let's take this.

[derrickstolee]

/azp run Microsoft.git
/azp run Microsoft.git (Windows (MSVC) Test 6)

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