This change makes it possible to run 'git cl' from
a Command Prompt using only the auto-installed git
from depot_tools.  It is needed because in MSysGit,
'git cl' invokes 'git' which calls 'git-cl'.  While
'git-cl' is pulled from depot_tools (since it's in
PATH), depot_tools/python_bin is not in PATH, and
so we execute git_cl.py through our auto-installed
python.exe directly.

update_depot_tools is fixed so that if we find
the auto-installed svn and git in depot_tools, we
execute those directly, otherwise falling back to
calling them based on PATH.

In both git-cl and update_depot_tools, we only enable
this new behavior if uname in PATH reports a 'MINGW'
environment.  This should not trigger on any other
environment where uname exists, including Cygwin which
should report 'CYGWIN'.
Review URL: http://codereview.chromium.org/7020035

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@87510 0039d316-1c4b-4281-b951-d872f2087c98