Implement ANALYZE

This commit makes it possible to run `ANALYZE` on a git foreign data
wrapper.  Lots of moving parts, here are the main improvements:

  * Kills global variables: I didn't know how to pass options when I
    started implementing this FDW... whoops!
  * Extracts a way to iterate over repository: adds a `walkRepository`
    function that takes an initial state (that can be of any type) and a
    pointer to a callback function.  This function will take a data
    structure that either contains an error code if the lookup failed,
    or a commit if the lookup succeeded.
  * Adds ways to gather statistics.  The costs estimates seem accurate
    enough for now but as libgit2 provides different ways to access
    repositories, it's not perfect (but can it be?)

A refactoring would be welcome at this point as the main C file is quite
large.  This will be done later.

Closes #24.

Author: franckverrot