commit-graph: when incompatible with graphs, indicate why

When `gc.writeCommitGraph = true`, it is possible that the commit-graph
is _still_ not written: replace objects, grafts and shallow repositories
are incompatible with the commit-graph feature.

Under such circumstances, we need to indicate to the user why the
commit-graph was not written instead of staying silent about it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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