Isolate depot_tools update into a separate script.

update_depot_tools

+#!/bin/bash
+# Copyright (c) 2010 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.
+
+# This script will try to sync the bootstrap directories and then defer control.
+
+base_dir=$(dirname "$0")
+
+# Test git and git --version.
+function test_git {
+  local GITV="$(git --version)" || {
+    echo "git isn't installed, please install it"
+    exit 1
+  }
+
+  GITV="${GITV##* }"          # Only examine last word (i.e. version number)
+  local GITD=( ${GITV//./ } ) # Split version number into decimals
+  if ((GITD[0] < 1 || (GITD[0] == 1 && GITD[1] < 6) )); then
+    echo "git version is ${GITV}, please update to a version later than 1.6"
+    exit 1
+  fi
+}
+
+# Test git svn and git svn --version.
+function test_git_svn {
+  local GITV="$(git svn --version)" || {
+    echo "git-svn isn't installed, please install it"
+    exit 1
+  }
+
+  GITV="${GITV#* version }"   # git svn --version has extra output to remove.
+  GITV="${GITV% (svn*}"
+  local GITD=( ${GITV//./ } ) # Split version number into decimals
+  if ((GITD[0] < 1 || (GITD[0] == 1 && GITD[1] < 6) )); then
+    echo "git version is ${GITV}, please update to a version later than 1.6"
+    exit 1
+  fi
+}
+
+
+# Update git checkouts prior the cygwin check, we don't want to use msysgit.
+if [ "X$DEPOT_TOOLS_UPDATE" != "X0" -a -e "$base_dir/.git" ]
+then
+  cd $base_dir
+  test_git_svn
+  # work around a git-svn --quiet bug
+  OUTPUT=`git svn rebase -q -q`
+  if [[ ! "$OUTPUT" =~ Current.branch ]]; then
+    echo $OUTPUT
+  fi
+  cd - > /dev/null
+fi
+
+# Use the batch file as an entry point if on cygwin.
+if [ "${OSTYPE}" = "cygwin" -a "${TERM}" != "xterm" ]; then
+   ${base_dir}/gclient.bat "$@"
+   exit
+fi
+
+
+# We're on POSIX (not cygwin). We can now safely look for svn checkout.
+if [ "X$DEPOT_TOOLS_UPDATE" != "X0" -a -e "$base_dir/.svn" ]
+then
+  # Update the bootstrap directory to stay up-to-date with the latest
+  # depot_tools.
+  svn -q up "$base_dir"
+fi