Add git-number script to calculate generation numbers for commits.

Compatible with any git topology (multiple roots, weird branching/merging, etc.)
I can't get it to be any faster (in python). Suggestions welcome :).

On z600/linux, this takes 5.1s to calculate the initial count for 2e3de954ef0a
(HEAD on src.git at the time of writing). Subsequent lookups take ~0.06s. For
reference, this machine takes 3s to just list the revisions in sorted order
without any additional processing (using rev-list).

All calculations are stored in a git-notes-style ref with the exception that the
leaf 'tree' object which would normally be stored in a git-notes world is
replaced with a packed binary file which consists of records [hash int]. Each
run of this script will create only 1 commit object on this internal ref which
will have as its parents:
  * The previous git number commit
  * All of the target commits we calculated numbers for.
This ref is then excluded on subsequent invocations of rev-list, which means that
git-number will only ever process commit objects which it hasn't already
calculated a value for. It also prevents you from attempting to number this
special ref :).

This implementation only has a 1-byte fanout which seems to be the best
performance for the repos we're dealing with (i.e. on the order of 500k commit
objects).  Bumping this up to a 2-byte fanout became extremely slow (I suspect
the internal caching structures I'm using are not efficient in this mode and
could be improved). Using no fanout is slower than the 1 byte fanout for lookups
by about 30%.

R=agable@chromium.org, stip@chromium.org, szager@chromium.org
BUG=280154,309692,skia:1639

Review URL: https://codereview.chromium.org/26109002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@236035 0039d316-1c4b-4281-b951-d872f2087c98

git-number

@@ -0,0 +1,27 @@
+#!/bin/sh
+# Copyright 2013 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# git-number -- a git-command for calculating and displaying the generation
+# number of a commit.
+
+# Test if this script is running under a MSys install.  If it is, we will
+# hardcode the path to Python where possible.
+OUTPUT="$(uname | grep 'MINGW')"
+MINGW=$?
+
+if [ $MINGW = 0 ]; then
+  base_dir="${0%\\*}"
+else
+  base_dir=$(dirname "$0")
+fi
+
+# Uncomment this line if you never use gclient.
+# "$base_dir"/update_depot_tools
+
+if [ -e "$base_dir/python.bat" -a $MINGW = 0 ]; then
+  PYTHONDONTWRITEBYTECODE=1 cmd.exe //c "$base_dir\\python.bat" "$base_dir\\git_number.py" "$@"
+else
+  PYTHONDONTWRITEBYTECODE=1 exec "$base_dir/git_number.py" "$@"
+fi