Might need newer git on ubuntu 16.04?!

[DanKegel]

This has happened on four different ubuntu 16.04 machines now:

Error: Command 'git checkout --quiet 7093b6561585bf66c0d44fe24e3f8235c263d979' returned non-zero exit status 1 in /mnt/home/buildbot/test.tmp/tmp/downloadedsrc/chromium/src/v8
error: Your local changes to the following files would be overwritten by checkout:
tools/turbolizer/turbolizer.png

Script:

export CEF_USE_GN=1
export GN_DEFINES="ffmpeg_branding=Chrome is_official_build=true proprietary_codecs=true remove_webcore_debug_symbols=true symbol_level=1 use_allocator=none use_sysroot=true"
export GYP_DEFINES="buildtype=Official disable_nacl=1 use_allocator=none use_sysroot=true"
export CEF_ARCHIVE_FORMAT=tar.bz2
rm -f automate-git.py
wget https://bitbucket.org/chromiumembedded/ ... ate-git.py
python automate-git.py --minimal-distrib --client-distrib --x64-build --build-target=cefsimple --force-clean --download-dir=$(pwd)/tmp/downloadedsrc --branch=3396
...
Previous HEAD position was d23e330... Updating trunk VERSION from 3470.0 to 3471.0
HEAD is now at 3dee21b... Publish DEPS for Chromium 67.0.3396.79
--> Chromium DEPS file: /mnt/home/buildbot/test.tmp/tmp/downloadedsrc/chromium/src/DEPS
-------- Running "gclient sync --reset --with_branch_heads --disable-syntax-validation --jobs 16" in "/mnt/home/buildbot/test.tmp/tmp/downloadedsrc/chromium"...
Syncing projects: 100% (103/103) src/third_party/deqp/src

src/v8 (ERROR)
----------------------------------------
[0:00:04] Started.
----------------------------------------
Error: Command 'git checkout --quiet 7093b6561585bf66c0d44fe24e3f8235c263d979' returned non-zero exit status 1 in /mnt/home/buildbot/test.tmp/tmp/downloadedsrc/chromium/src/v8
error: Your local changes to the following files would be overwritten by checkout:
tools/turbolizer/turbolizer.png
Please, commit your changes or stash them before you can switch branches.
Aborting

Traceback (most recent call last):
File "automate-git.py", line 1397, in <module>
('--reset ' if discard_local_changes else ''), chromium_dir, depot_tools_dir)
File "automate-git.py", line 61, in run
args, cwd=working_dir, env=env, shell=(sys.platform == 'win32'))
File "/usr/lib/python2.7/subprocess.py", line 541, in check_call
raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['gclient', 'sync', '--reset', '--with_branch_heads', '--disable-syntax-validation', '--jobs', '16']' returned non-zero exit status 1

[DanKegel]

Reduced test case somewhat and filed https://bitbucket.org/chromiumembedded/ ... buntu-1604

[DanKegel]

Installing a newer git than the one that ships with ubuntu 16.04 seems to work around the problem. e.g.

sudo add-apt-repository ppa:git-core/ppa
sudo apt-get update
sudo apt-get install git