do not create "struct commit" with xcalloc

In both blame and merge-recursive, we sometimes create a
"fake" commit struct for convenience (e.g., to represent the
HEAD state as if we would commit it). By allocating
ourselves rather than using alloc_commit_node, we do not
properly set the "index" field of the commit. This can
produce subtle bugs if we then use commit-slab on the
resulting commit, as we will share the "0" index with
another commit.

We can fix this by using alloc_commit_node() to allocate.
Note that we cannot free the result, as it is part of our
commit allocator. However, both cases were already leaking
the allocated commit anyway, so there's nothing to fix up.

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

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