commit-graph: fix bug around octopus merges

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>

commit-graph.c

@@ -1629,7 +1629,7 @@ static void sort_and_scan_merged_commits(struct write_commit_graph_context *ctx)
 				num_parents++;
 
 			if (num_parents > 2)
-				ctx->num_extra_edges += num_parents - 2;
+				ctx->num_extra_edges += num_parents - 1;
 		}
 	}
 

t/t5324-split-commit-graph.sh

@@ -319,7 +319,9 @@ test_expect_success 'add octopus merge' '
 	git merge commits/3 commits/4 &&
 	git branch merge/octopus &&
 	git commit-graph write --reachable --split &&
-	git commit-graph verify &&
+	git commit-graph verify 2>err &&
+	test_line_count = 3 err &&
+	test_i18ngrep ! warning err &&
 	test_line_count = 3 $graphdir/commit-graph-chain
 '