Watchman support for git

[dturner@twopensource.com]

The most sigificant patch uses Facebook's watchman daemon[1] to monitor
the repository work tree for changes.  This makes allows git status
to avoid traversing the entire work tree to find changes.

This change requires libwatchman[2], a client library that I wrote for
watchman.

While making the watchman change, I also made a change to the index
format (contributed here in a separate patch).  Index integrity
checking uses the same SHA1 algorithm as the rest of git; this is
actually relatively slow.  It's not a huge part of run-time, but since
I wanted to do the same checking for the Watchman filesystem cache
(which is about twice as large as the index), I decided to optimize it
anyway.  I switched to VMAC.  VMAC is supposed to be a MAC, but
there's no reason it can't be used with a fixed key as a simple
integrity check.  VMAC is roughly five times faster than SHA1 on my
machine; This adds up to a 5% overal speed improvement on git status
(depending on the structure of your repo, and about 15% on git diff
--cached with no cached changes).

The index format change might be less important with the split index;
I haven't investigated that since at the time I wrote these patches,
it didn't exist.

Some numbers follow.  They are on my laptop, which has 4x i5-2520M
processors, 8GB of RAM, and a solid state disk.  They're all tested
with a hot cache.

Test repository 1: Linux

Linux is about 45k files in 3k directories.  The average length of a
filename is about 32 bytes.

Git status timing:
no watchman: 125ms
watchman: 90ms

Test repository 2: Superscience

My second test repository (which is a semi-synthetic repo generated
from various Twitter internal repos) is somewhat larger than this, and
gets a correspondingly larger improvement.  It is about 65k files in
20k directories; the average length of a filename is 67 bytes.

Git status timing:
no watchman, index version 4: 370 ms
no watchman, index version 5: 365 ms
watchman, index version 4: 170 ms
watchman, index version 5: 165 ms


[1] https://github.com/facebook/watchman
[2] https://github.com/twitter/libwatchman