Add GitGutterFold command.

Closes #286, #571.

autoload/gitgutter/fold.vim

@@ -0,0 +1,52 @@
+function! gitgutter#fold#enable()
+  call s:save_fold_state()
+
+  setlocal foldexpr=gitgutter#fold#level(v:lnum)
+  setlocal foldmethod=expr
+  setlocal foldlevel=0
+  setlocal foldenable
+
+  call gitgutter#utility#setbufvar(bufnr(''), 'folded', 1)
+endfunction
+
+
+function! gitgutter#fold#disable()
+  call s:restore_fold_state()
+  call gitgutter#utility#setbufvar(bufnr(''), 'folded', 0)
+endfunction
+
+
+function! gitgutter#fold#toggle()
+  if s:folded()
+    call gitgutter#fold#disable()
+  else
+    call gitgutter#fold#enable()
+  endif
+endfunction
+
+
+function! gitgutter#fold#level(lnum)
+  return gitgutter#hunk#in_hunk(a:lnum) ? 0 : 1
+endfunction
+
+
+function! s:save_fold_state()
+  call gitgutter#utility#setbufvar(bufnr(''), 'foldmethod', &foldmethod)
+  if &foldmethod ==# 'manual'
+    mkview
+  endif
+endfunction
+
+function! s:restore_fold_state()
+  let &foldmethod = gitgutter#utility#getbufvar(bufnr(''), 'foldmethod')
+  if &foldmethod ==# 'manual'
+    loadview
+  else
+    normal! zx
+  endif
+endfunction
+
+function! s:folded()
+  return gitgutter#utility#getbufvar(bufnr(''), 'folded')
+endfunction
+