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
+

autoload/gitgutter/hunk.vim

@@ -123,6 +123,30 @@ function! gitgutter#hunk#cursor_in_hunk(hunk) abort
   return 0
 endfunction
 
+
+function! gitgutter#hunk#in_hunk(lnum)
+  " Hunks are sorted in the order they appear in the buffer.
+  for hunk in gitgutter#hunk#hunks(bufnr(''))
+    " if in a hunk on first line of buffer
+    if a:lnum == 1 && hunk[2] == 0
+      return 1
+    endif
+
+    " if in a hunk generally
+    if a:lnum >= hunk[2] && a:lnum < hunk[2] + (hunk[3] == 0 ? 1 : hunk[3])
+      return 1
+    endif
+
+    " if hunk starts after the given line
+    if a:lnum < hunk[2]
+      return 0
+    endif
+  endfor
+
+  return 0
+endfunction
+
+
 function! gitgutter#hunk#text_object(inner) abort
   let hunk = s:current_hunk()