[PATCH/RFC 0/6] pack-objects hook for upload-pack

I've often wanted to intercept the call from upload-pack to pack-objects. The final patch in this series goes into more detail, but basically it's good for:

  1. Capturing the output from pack-objects for debugging/inspection.

  2. Capturing the input to pack-objects to replay for debugging or
     performance analysis.

  3. Caching pack-objects output.

It's pretty trivial to add a hook to run instead of pack-objects (and the hook would just run pack-objects itself). But we don't want to run hooks in upload-pack, because we don't necessarily trust the repository we're running in.

So instead we'd have to learn about the hook from the environment, or from any of the non-repo config files. And that in turn requires some support from the config code, which is what patches 1-5 are doing (along with some cleanups along the way).

I think the config refactoring is all pretty sane, and could support other "dangerous" features besides this particular hook in a way that's easy to use for site admins (whether they trust the repos they're serving or not). I've marked this "RFC" because I'd like input on some details of the final patch; I'll point out my questions separately there.

  [1/6]: git_config_with_options: drop "found" counting
  [2/6]: git_config_parse_parameter: refactor cleanup code
  [3/6]: config: set up config_source for command-line config
  [4/6]: config: return configset value for current_config_ functions
  [5/6]: config: add a notion of "scope"
  [6/6]: upload-pack: provide a hook for running pack-objects

-Peff