EDITOR environment variable shouldn't be required anymore
can contain quotes and multiple arguments.

Uses shell=True to let the shell parse the quoted string
instead of handling all forms of quotes ourselves. 

By default, msysgit ships with the TERM environment variable
set, when set to 'msys', we prepend 'env' to the executed
command. That is required so msys knows which program
to run the given file.

BUG=none
TEST=On many platforms. Win7, WinXP, Ubuntu.
Review URL: http://codereview.chromium.org/6685062

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@78332 0039d316-1c4b-4281-b951-d872f2087c98