Before this commit some Windows users saw the command prompt pop
up briefly, and/or the taskbar flicker, every time the plugin ran.
Now the plugin will use xolox's vim-shell and vim-misc, if they are
available and we are on Windows, to execute external commands. Xolox's
clever plugins avoid the command prompt popup and taskbar flicker.
Windows users with those plugins installed can opt out by setting a
variable in their vimrc.
Many thanks to @suxpert for the initial code.
Note that line highlighting requires signs to be placed (because the
line highlight is simply an attribute of a sign). If the user doesn't
want to see signs in the sign column, but does want line highlighting,
then we make the signs in the sign column invisible.
If neither the signs in the sign column nor the line highlights are
needed (presumably the user just looks at the hunks stats) then we can
remove all signs, at which point Vim removes the sign column...unless
the "sign column always" option is set.
