[RFC/PATCH 0/4] config include directives

This series provides a way for config files to include other config
files in two ways:

  1. From other files in the filesystem. This is implemented by patch 1
     below, and is hopefully straightforward and uncontroversial.  See
     that patch for more rationale.

  2. From blobs in the repo. This is implemented by patch 4, with
     patches 2 and 3 providing the necessary refactoring. This
     is one way of implementing the often asked-for "respect shared
     config inside the repo" feature, but attempts to mitigate some of
     the security concerns. The interface for using it safely is a bit
     raw, but I think it's a sane building block, and somebody could
     write a fancier shared-config updater on top of it if they wanted
     to.

  [1/4]: config: add include directive
  [2/4]: config: factor out config file stack management
  [3/4]: config: support parsing config data from buffers
  [4/4]: config: allow including config from repository blobs

-Peff