commit: fix duplication regression in permission error output

commit: fix duplication regression in permission error output

Fix a regression in the error output emitted when .git/objects can't
be written to. Before 9c4d6c0297b (cache-tree: Write updated
cache-tree after commit, 2014-07-13) we'd emit only one "insufficient
permission" error, now we'll do so again.

The cause is rather straightforward, we've got WRITE_TREE_SILENT for
the use-case of wanting to prepare an index silently, quieting any
permission etc. error output. Then when we attempt to update to
that (possibly broken) index we'll run into the same errors again.

But with 9c4d6c0297b the gap between the cache-tree API and the object
store wasn't closed in terms of asking write_object_file() to be
silent. I.e. post-9c4d6c0297b the first call is to prepare_index(),
and after that we'll call prepare_to_commit(). We only want verbose
error output from the latter.

So let's add and use that facility with a corresponding HASH_SILENT
flag, its only user is cache-tree.c's update_one(), which will set it
if its "WRITE_TREE_SILENT" flag is set.

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