Status update on merge-recursive in C

Status update on merge-recursive in C

This is just an update for people being interested. Alex and me were
busy with that project for a few days now. While it has progressed nicely,
there are quite a couple TODOs in merge-recursive.c, just search for "TODO".

For impatient people: yes, it passes all the tests, and yes, according
to the evil test Alex did, it is faster than the Python script.

But no, it is not yet finished. Biggest points are:

- there are still three external calls
- in the end, it should not be necessary to write the index more than once
  (just before exiting)
- a lot of things can be refactored to make the code easier and shorter

BTW we cannot just plug in git-merge-tree yet, because git-merge-tree
does not handle renames at all.

This patch is meant for testing, and as such,

- it compile the program to git-merge-recur
- it adjusts the scripts and tests to use git-merge-recur instead of
  git-merge-recursive
- it provides "TEST", a script to execute the tests regarding -recursive
- it inlines the changes to read-cache.c (read_cache_from(), discard_cache()
  and refresh_cache_entry())

Brought to you by Alex Riesen and Dscho

Signed-off-by: Junio C Hamano <junkio@cox.net>