vimrc: Regenerate binary spell files on startup

Enables the "union" merge strategy for spell files and regenerates the
binary .spl files when modified by a merge.

.gitattributes

@@ -0,0 +1 @@
+*.add merge=union

.vimrc

@@ -130,6 +130,13 @@ endif
 " Remove object files from file globs
 set wildignore+=*.o
 
+" Regenerate binary spell files if the .add files were modified
+for d in glob('~/.vim/spell/*.add', 1, 1)
+    if filereadable(d) && (!filereadable(d . '.spl') || getftime(d) > getftime(d . '.spl'))
+        exec 'mkspell! ' . fnameescape(d)
+    endif
+endfor
+
 " Show a tree-style netrw listing
 let g:netrw_home = '~/.netrw'
 let g:netrw_liststyle = 3

vimfiles/spell/.gitignore

@@ -0,0 +1 @@
+*.spl