Add GitGutterFold command.

Closes #286, #571.

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()