odb_mkstemp: use git_path_buf

odb_mkstemp: use git_path_buf

Since git_path_buf() is smart enough to replace "objects/"
with the correct object path, we can use it instead of
manually assembling the path. That's slightly shorter, and
will clean up any non-canonical bits in the path.

Signed-off-by: Jeff King <peff@peff.net>