[RFC PATCH 0/2] Add support for multi threaded checkout

There was some discussion a while back about improving git performance on NFS (http://article.gmane.org/gmane.comp.version-control.git/100950). This led to Linus adding the 'preload_index' function, which improves performance of several commands using multi threading. He also briefly described how to do the same for 'git checkout'. Well, I finally found some time to work on it, and will post patches shortly. This is my first patch; apologies if I screwed something up.

Patch 1 adds the functionality, and 2 adds a config option to enable/disable it.

Much of the patch is literally copy/paste from preload-index.c into unpack-trees.c. Many of the functions called during checkout are not thread safe, so I added a mutex in entry.c to serialize basically everything except writing the files to disk. I also added a mutex in unpack-trees.c for the progress meter.

It passes the test suite, and seems fairly safe to my naïve eyes.

Here are some benchmarks, cloning a linux kernel repo I had on an NFS drive:

                   NFS->NFS    NFS->Local
master (53682f0c)    2:46.1          13.3
with threads           36.6          18.2

So it improved performance on NFS significantly. Unfortunately it also degraded performance on the local disk significantly. I'm hoping someone will suggest a way to mitigate that... I think it would be reasonable to disable the threading except when the work dir is on NFS, but I don't know how to detect that. Even in that case it will have *some* impact from locking/unlocking the mutex, but I think it would be in the noise.

James