Make git-cl and update_depot_tools msys-compatible.

Make git-cl and update_depot_tools msys-compatible.

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'.
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=87510

[cmp, 2011-06-01 18:32:34]

I plan to land this again today.  Will stay alert for reports of trouble.

[nsylvain, 2011-06-01 18:51:22]

LGTM

[M-A Ruel, 2011-06-01 18:52:28]

lgtm with cleanup

http://codereview.chromium.org/7020035/diff/1/update_depot_tools
File update_depot_tools (right):

http://codereview.chromium.org/7020035/diff/1/update_depot_tools#newcode36
update_depot_tools:36: function test_git {
Oh BTW, () is missing here and on line 51. But in any case test_git is not used
at all.