user-defined g:ref_cache_dir should also be expanded

[y]

The current behavior is to only expand the default directory:

if !exists('g:ref_cache_dir')
  let g:ref_cache_dir = expand('~/.vim_ref_cache')
endif

But it should also call expand on the user-defined directory:

if !exists('g:ref_cache_dir')
  let g:ref_cache_dir = '~/.vim_ref_cache'
endif
let g:ref_cache_dir = expand(g:ref_cache_dir)

This is the expected behavior in vim and other plugins.

[thinca]

I think this behavior differs with plugin. For example, netrw doesn't expand g:netrw_home.