Sometimes you want to create a commit but not write it out to the
objectdb immediately. For these cases, provide a new function to
retrieve the buffer instead of having to go through the db.

Coverity fixes

The `giterr_set_str` does not actually honor `GITERR_OS`.  Remove
the documentation that claims that we do.

The overflow check in `read_reuc` tries to verify if the
`git__strtol32` parses an integer bigger than UINT_MAX. The `tmp`
variable is casted to an unsigned int for this and then checked
for being greater than UINT_MAX, which obviously can never be
true.

Fix this by instead fixing the `mode` field's size in `struct
git_index_reuc_entry` to `uint32_t`. We can now parse the int
with `git__strtol64`, which can never return a value bigger than
`UINT32_MAX`, and additionally checking if the returned value is
smaller than zero.

We do not need to handle overflows explicitly here, as
`git__strtol64` returns an error when the returned value would
overflow.

Filter registration

We should be checking whether the object we're looking up is a commit,
and we should let the caller know whether the not-found return code
comes from a bad object type or just a missing signature.