Use match window opts to set initial window height

[mmrwoods]

Use the min height of the match window as the initial height of the
ControlP window, rather than always using 1 as the starting height.

This is primarily to workaround MacVim rendering glitches triggered by
automagic resizing of windows when in non-native fullscreen mode, and
core text renderer is enabled, but doesn't seem an unreasonable default.

When MacVim is running in non-native fullscreen mode with core text
renderer enabled, automatically resizing windows causes some parts of
the screen to become blacked out. This is a known issue with MacVim...
http://vim.1045645.n5.nabble.com/Trouble-with-the-Core-Text-renderer-in-non-native-full-screen-td5713348.html

Allowing the ControlP window to open with a fixed height, by setting the
min and max match window height to the same value, makes it trivial to
work around the rendering glitches using the g:ctrlp_buffer_func option
to clear and redraw the screen when exiting the ControlP buffer, e.g.

" Clear and redraw screen when exiting CtrlP buffer
let g:ctrlp_buffer_func = {
  \ 'exit':  'RedrawScreen',
  \ }
function RedrawScreen()
  exec 'redraw!'
endfunction

Before this change, the only solution I could find to the need to redraw
after the ControlP window was resized to match window min height was a
fugly CursorMoved autocmd to redraw the screen the first time the cursor
moves inside the ControlP window, e.g.

" Fix the dimensions of the CtrlP window to avoid dynamic resizing
let g:ctrlp_match_window = 'bottom,order:btt,min:10,max:10,results:10'

" Redraw screen when CtrlP window is resized after scanning for files
" This is only necessary because CtrlP doesn't open the window using
" the dimensions set above, but resizes *after* scanning for files.
autocmd CursorMoved ControlP
  \ if !exists('b:ctrlp_did_redraw') |
  \   exec 'redraw!' |
  \   let b:ctrlp_did_redraw = 1 |
  \ endif

This commit allows me to remove that fugly CursorMoved autocmd, which
makes me super happy :-)